10. Consider the function $f(x, y, z) = xy^2z$ where $x > 0$, $y > 0$, and $z > 0$.
a. Find the derivative of $f$ at the point $(1,2,3)$ in the direction of $(4,5,6)$.
b. In what direction is $f$ increasing the fastest at the point $(1,2,3)$?
c. What is the maximum rate of change of $f$ at the point $(1,2,3)$?