Find the Specified Domain $( f g ) ( x )$

Multiple Choice

Find the specified domain.
-Find the domain of $( f g ) ( x )$ when $f ( x ) = \sqrt { 6 x + 9 }$ and $g ( x ) = \sqrt { 7 x - 9 }$

A) $[ 0 , \infty )$
B) $( - \infty , \infty )$
C) $\left[ - \frac { 9 } { 7 } , \infty \right)$
D) $\left[ \frac { 9 } { 7 } , \infty \right)$