Find the Directional Derivative of the Function $f ( x , y ) = ( x + 6 ) e ^ { y }$

Multiple Choice

Find the directional derivative of the function $f ( x , y ) = ( x + 6 ) e ^ { y }$ at the point $P ( 7,0 )$ in the direction of the unit vector that makes the angle $\theta = \frac { \pi } { 2 }$ with the positive $x$ -axis.

A) $6 e ^ { 7 }$
B) 13
C) 1
D) $e ^ { 7 }$