Consider the following.
f(x, y, z) = x\textsuperscript{2}yz - xyz\textsuperscript{3}, P(2, -1, 1), u = \left<0, \frac{4}{5}, -\frac{3}{5}\right>
(a) Find the gradient of f.
\nabla f(x, y, z) =
(b) Evaluate the gradient at the point P.
\nabla f(2, -1, 1) =
(c) Find the rate of change of f at P in the direction of the vector u.
D\textsubscript{u}f(2, -1, 1) =