Evaluate the Polynomial Function $f ( x ) = x ^ { 2 } + 6 x$

Multiple Choice

Evaluate the polynomial function.
-If $f ( x ) = x ^ { 2 } + 6 x$ , find $f ( a )$ .

A) $x ^ { 2 } + a x$
B) $a x ^ { 2 } + 6 a x$
C) $6 a$
D) $a ^ { 2 } + 6 a$