Question

Compute the rate of change of f(x,y,z) = xy + yz^2 +xz^3 at P (2,0,3) in the direction of the unit vector u = (-2/3,-1/3,2/3). In addition, find the maximum rate of change of f at P and the direction in which it occurs. I can get the directional derivative but I am uncertain about the second half of the problem.