Find the directional derivative of fxyz in the direction of

Find the directional derivative of f(x,y.z) = in the direction of b = -i + 2j + 4k at the point (1.2, -1).

gradient vector is

2x y e^(z/2) i + x^2 e ^(z/2) j + 1/2 x^2 ye^(z/2) z
At (1,2,-1) we have
4 e^(-1/2) i + e^(-1/2)j + e^(-1/2) k

In the direction of - i +2j + 4k
We have (1+2^2 +4^2)= 21
So a unit vector is -1/21i +2/21 j + 4/21 k

Directional derivative is thus the dot product of the two vectors