Directional derivative problem

[karens]

nvmnd - solved

 

[Coto]

Hey karens,

Well you have the formula written above. Why don't you start by showing us what $$f_x (2,1), f_y (2,1)$$ are and what $$a, b$$ should be considering your unit vector $$\left(\frac{3}{5}, \frac{-4}{5}\right)$$.