<DOCUMENT>
<TYPE>EX-24
<SEQUENCE>2
<FILENAME>mg3-ex24.txt
<TEXT>

                                                                 Exhibit 24

                             POWER OF ATTORNEY


     KNOW ALL PERSONS BY THESE PRESENTS that GS CAPITAL PARTNERS VI FUND,
L.P. (the "Company") does hereby make, constitute and appoint each of Bruce
A. Albert, Andrea Louro DeMar, Yvette Kosic, Rachel E. Parrish, Michael T.
Seeley, and Kevin P. Treanor, (and any other employee of The Goldman Sachs
Group, Inc. or one of its affiliates designated in writing by one of the
attorneys-in-fact), acting individually, its true and lawful attorney, to
execute and deliver in it name and on its behalf whether the Company is
acting individually or as representative of others, any and all filings
required to be made by the Company under the Securities Exchange Act of
1934, (as amended, the "Act"), with respect to securities which may be
deemed to be beneficially owned by the Company under the Act, giving and
granting unto each said attorney-in-fact power and authority to act in the
premises as fully and to all intents and purposes as the Company might or
could do if personally present by one of its authorized signatories, hereby
ratifying and confirming all that said attorney-in-fact shall lawfully do
or cause to be done by virtue hereof.

     THIS POWER OF ATTORNEY shall remain in full force and effect until
either revoked in writing by the undersigned or until such time as the
person or persons to whom power of attorney has been hereby granted
cease(s) to be an employee of The Goldman Sachs Group, Inc. or one of its
affiliates.

     IN WITNESS WHEREOF, the undersigned has duly subscribed these presents
as of April 1, 2008.

GS CAPITAL PARTNERS VI FUND, L.P.,
By:    GSCP VI Advisors, L.L.C.,
       its general partner

By:     /s/Christine Vollertsen
        --------------------------------
Name:   Christine Vollertsen
Title:  Vice President


<PAGE>

                             POWER OF ATTORNEY

     KNOW ALL PERSONS BY THESE PRESENTS that GS CAPITAL PARTNERS VI
OFFSHORE FUND, L.P. (the "Company") does hereby make, constitute and
appoint each of Bruce A. Albert, Andrea Louro DeMar, Yvette Kosic, Rachel
E. Parrish, Michael T. Seeley, and Kevin P. Treanor, (and any other
employee of The Goldman Sachs Group, Inc. or one of its affiliates
designated in writing by one of the attorneys-in-fact), acting
individually, its true and lawful attorney, to execute and deliver in it
name and on its behalf whether the Company is acting individually or as
representative of others, any and all filings required to be made by the
Company under the Securities Exchange Act of 1934, (as amended, the "Act"),
with respect to securities which may be deemed to be beneficially owned by
the Company under the Act, giving and granting unto each said
attorney-in-fact power and authority to act in the premises as fully and to
all intents and purposes as the Company might or could do if personally
present by one of its authorized signatories, hereby ratifying and
confirming all that said attorney-in-fact shall lawfully do or cause to be
done by virtue hereof.

     THIS POWER OF ATTORNEY shall remain in full force and effect until
either revoked in writing by the undersigned or until such time as the
person or persons to whom power of attorney has been hereby granted
cease(s) to be an employee of The Goldman Sachs Group, Inc. or one of its
affiliates.

     IN WITNESS WHEREOF, the undersigned has duly subscribed these presents
as of April 1, 2008.

GS CAPITAL PARTNERS VI OFFSHORE FUND, L.P.,
By:     GSCP VI Offshore Advisors, L.L.C.,
        its general partner

By:     /s/Christine Vollertsen
        --------------------------------
Name:   Christine Vollertsen
Title:  Vice President


<PAGE>

                             POWER OF ATTORNEY

     KNOW ALL PERSONS BY THESE PRESENTS that GS CAPITAL PARTNERS VI
PARALLEL, L.P. (the "Company") does hereby make, constitute and appoint
each of Bruce A. Albert, Andrea Louro DeMar, Yvette Kosic, Rachel E.
Parrish, Michael T. Seeley, and Kevin P. Treanor, (and any other employee
of The Goldman Sachs Group, Inc. or one of its affiliates designated in
writing by one of the attorneys-in-fact), acting individually, its true and
lawful attorney, to execute and deliver in it name and on its behalf
whether the Company is acting individually or as representative of others,
any and all filings required to be made by the Company under the Securities
Exchange Act of 1934, (as amended, the "Act"), with respect to securities
which may be deemed to be beneficially owned by the Company under the Act,
giving and granting unto each said attorney-in-fact power and authority to
act in the premises as fully and to all intents and purposes as the Company
might or could do if personally present by one of its authorized
signatories, hereby ratifying and confirming all that said attorney-in-fact
shall lawfully do or cause to be done by virtue hereof.

     THIS POWER OF ATTORNEY shall remain in full force and effect until
either revoked in writing by the undersigned or until such time as the
person or persons to whom power of attorney has been hereby granted
cease(s) to be an employee of The Goldman Sachs Group, Inc. or one of its
affiliates. I

     N WITNESS WHEREOF, the undersigned has duly subscribed these presents
as of April 1, 2008.

GS CAPITAL PARTNERS VI PARALLEL, L.P.
By:     GS Advisors VI, L.L.C.,
        its general partner

By:     /s/Christine Vollertsen
        --------------------------------
Name:   Christine Vollertsen
Title:  Vice President

<PAGE>

                             POWER OF ATTORNEY

     KNOW ALL PERSONS BY THESE PRESENTS that GS CAPITAL PARTNERS VI GmbH &
CO. KG (the "Company") does hereby make, constitute and appoint each of
Bruce A. Albert, Andrea Louro DeMar, Yvette Kosic, Rachel E. Parrish,
Michael T. Seeley, and Kevin P. Treanor, (and any other employee of The
Goldman Sachs Group, Inc. or one of its affiliates designated in writing by
one of the attorneys-in-fact), acting individually, its true and lawful
attorney, to execute and deliver in it name and on its behalf whether the
Company is acting individually or as representative of others, any and all
filings required to be made by the Company under the Securities Exchange
Act of 1934, (as amended, the "Act"), with respect to securities which may
be deemed to be beneficially owned by the Company under the Act, giving and
granting unto each said attorney-in-fact power and authority to act in the
premises as fully and to all intents and purposes as the Company might or
could do if personally present by one of its authorized signatories, hereby
ratifying and confirming all that said attorney-in-fact shall lawfully do
or cause to be done by virtue hereof.

     THIS POWER OF ATTORNEY shall remain in full force and effect until
either revoked in writing by the undersigned or until such time as the
person or persons to whom power of attorney has been hereby granted
cease(s) to be an employee of The Goldman Sachs Group, Inc. or one of its
affiliates.

     IN WITNESS WHEREOF, the undersigned has duly subscribed these presents
as of April 1, 2008.

GS CAPITAL PARTNERS VI GmbH & CO. KG
By:     GS Advisors VI, L.L.C.,
        its managing limited partner

By:     /s/Christine Vollertsen
        --------------------------------
Name:   Christine Vollertsen
Title:  Vice President


<PAGE>

                             POWER OF ATTORNEY

     KNOW ALL PERSONS BY THESE PRESENTS that GSCP VI ADVISORS, L.L.C. (the
"Company") does hereby make, constitute and appoint each of Bruce A.
Albert, Andrea Louro DeMar, Yvette Kosic, Rachel E. Parrish, Michael T.
Seeley, and Kevin P. Treanor, (and any other employee of The Goldman Sachs
Group, Inc. or one of its affiliates designated in writing by one of the
attorneys-in-fact), acting individually, its true and lawful attorney, to
execute and deliver in it name and on its behalf whether the Company is
acting individually or as representative of others, any and all filings
required to be made by the Company under the Securities Exchange Act of
1934, (as amended, the "Act"), with respect to securities which may be
deemed to be beneficially owned by the Company under the Act, giving and
granting unto each said attorney-in-fact power and authority to act in the
premises as fully and to all intents and purposes as the Company might or
could do if personally present by one of its authorized signatories, hereby
ratifying and confirming all that said attorney-in-fact shall lawfully do
or cause to be done by virtue hereof.

     THIS POWER OF ATTORNEY shall remain in full force and effect until
either revoked in writing by the undersigned or until such time as the
person or persons to whom power of attorney has been hereby granted
cease(s) to be an employee of The Goldman Sachs Group, Inc. or one of its
affiliates.

     IN WITNESS WHEREOF, the undersigned has duly subscribed these presents
as of April 1, 2008.

GSCP VI ADVISORS, L.L.C.

By:     /s/Christine Vollertsen
        --------------------------------
Name:   Christine Vollertsen
Title:  Vice President

<PAGE>

                             POWER OF ATTORNEY

     KNOW ALL PERSONS BY THESE PRESENTS that GSCP VI OFFSHORE ADVISORS,
L.L.C. (the "Company") does hereby make, constitute and appoint each of
Bruce A. Albert, Andrea Louro DeMar, Yvette Kosic, Rachel E. Parrish,
Michael T. Seeley, and Kevin P. Treanor, (and any other employee of The
Goldman Sachs Group, Inc. or one of its affiliates designated in writing by
one of the attorneys-in-fact), acting individually, its true and lawful
attorney, to execute and deliver in it name and on its behalf whether the
Company is acting individually or as representative of others, any and all
filings required to be made by the Company under the Securities Exchange
Act of 1934, (as amended, the "Act"), with respect to securities which may
be deemed to be beneficially owned by the Company under the Act, giving and
granting unto each said attorney-in-fact power and authority to act in the
premises as fully and to all intents and purposes as the Company might or
could do if personally present by one of its authorized signatories, hereby
ratifying and confirming all that said attorney-in-fact shall lawfully do
or cause to be done by virtue hereof.

     THIS POWER OF ATTORNEY shall remain in full force and effect until
either revoked in writing by the undersigned or until such time as the
person or persons to whom power of attorney has been hereby granted
cease(s) to be an employee of The Goldman Sachs Group, Inc. or one of its
affiliates.

     IN WITNESS WHEREOF, the undersigned has duly subscribed these presents
as of April 1, 2008.

GSCP VI OFFSHORE ADVISORS, L.L.C.

By:     /s/Christine Vollertsen
        --------------------------------
Name:   Christine Vollertsen
Title:  Vice President

<PAGE>

                             POWER OF ATTORNEY

     KNOW ALL PERSONS BY THESE PRESENTS that GS ADVISORS VI, L.L.C. (the
"Company") does hereby make, constitute and appoint each of Bruce A.
Albert, Andrea Louro DeMar, Yvette Kosic, Rachel E. Parrish, Michael T.
Seeley, and Kevin P. Treanor, (and any other employee of The Goldman Sachs
Group, Inc. or one of its affiliates designated in writing by one of the
attorneys-in-fact), acting individually, its true and lawful attorney, to
execute and deliver in it name and on its behalf whether the Company is
acting individually or as representative of others, any and all filings
required to be made by the Company under the Securities Exchange Act of
1934, (as amended, the "Act"), with respect to securities which may be
deemed to be beneficially owned by the Company under the Act, giving and
granting unto each said attorney-in-fact power and authority to act in the
premises as fully and to all intents and purposes as the Company might or
could do if personally present by one of its authorized signatories, hereby
ratifying and confirming all that said attorney-in-fact shall lawfully do
or cause to be done by virtue hereof.

     THIS POWER OF ATTORNEY shall remain in full force and effect until
either revoked in writing by the undersigned or until such time as the
person or persons to whom power of attorney has been hereby granted
cease(s) to be an employee of The Goldman Sachs Group, Inc. or one of its
affiliates.

     IN WITNESS WHEREOF, the undersigned has duly subscribed these presents
as of April 1, 2008.

GS ADVISORS VI, L.L.C.

By:     /s/Christine Vollertsen
        --------------------------------
Name:   Christine Vollertsen
Title:  Vice President

</TEXT>
</DOCUMENT>
