Let $f ( x , y , z ) = x ^ { 3 } + 3 x z + 4 y z + z ^ { 2 }$

Essay

Let $f ( x , y , z ) = x ^ { 3 } + 3 x z + 4 y z + z ^ { 2 }$ .Find , where $\vec { v }$ is a unit vector in the direction of $- \vec { i } + \vec { j } - \vec { k }$ .