Compute the Gradient of the Function at the Given Point $$f ( x , y , z ) = \tan ^ { - 1 } \frac { 7 x } { 9 y + 3 z } , ( 9,0,0 )$$

Multiple Choice

Compute the gradient of the function at the given point.
- $$f ( x , y , z ) = \tan ^ { - 1 } \frac { 7 x } { 9 y + 3 z } , ( 9,0,0 )$$

A) ${\nabla} f = \frac { 1 } { 7 } \mathrm { j } - \frac { 1 } { 21 } \mathbf { k }$
B) ${\nabla} f = - \frac { 1 } { 21 } j + \frac { 1 } { 7 } k$
C) ${\nabla} f = - \frac { 1 } { 7 } \mathbf { j } - \frac { 1 } { 21 } \mathbf { k }$
D) ${\nabla} f = - \frac { 1 } { 21 } j \mathbf { j } - \frac { 1 } { 7 } k$