Evaluate the Function at the Indicated Value $f ( 6 )$

Evaluate the function at the indicated value.
-Find $f ( 6 )$ when $f ( x ) = \sqrt { x ^ { 2 } + 7 x }$ .

A) $\sqrt { 78 }$
B) $2 \sqrt { 14 }$
C) $\sqrt { 85 }$
D) $\sqrt { 43 }$