Find the directional derivative of the function f(x,y,z) =z^2 -xy^2

Question

at the point P(2,1,3) in the direction of the vector v=< -1, 2, 2 >.

Answer 1

df/du=2z dz/du -y^2 dx/du -2xy dy/du and at P
df/du=6 dz/du -dx/du -4 dy/du check that.
du/du= - dx/du -4 dy/du + 6 dz/du by puting them in order.
Now applying the directional v,
df/du= 1+8 +12=21 check that.