Add or Subtract as Indicated $9 \sqrt { 6 } - 2 \sqrt { 6 }$

Add or subtract as indicated.
- $9 \sqrt { 6 } - 2 \sqrt { 6 }$

A) $11 \sqrt { 6 }$
B) $7 \sqrt { 12 }$
C) $7 \sqrt { 6 }$
D) $- 18 \sqrt { 12 }$