When $f ( x ) = x + 6$ And $g ( x ) = 3 x ^ { 2 } + 3 x$

Multiple Choice

When $f ( x ) = x + 6$ and $g ( x ) = 3 x ^ { 2 } + 3 x$ , find $( f + g ) ( x )$ .

A) $( f + g ) ( x ) = 4 x ^ { 2 } + 3 x + 6$
B) $( f + g ) ( x ) = 4 x ^ { 3 } + 9 x ^ { 2 }$
C) $( f + g ) ( x ) = 4 x ^ { 2 } + 9 x$
D) $( f + g ) ( x ) = 3 x ^ { 2 } + 4 x + 6$