$(2)$ Consider the function f$($x$,$y$)=$x$^(2)($y$-1).$

$($a$)$ What is the directional derivative of f at $(1,3)$ in the direction of the vector

vec$($v$)=(3,-4)?$

$($b$)$ What is the maximum directional derivative of f at $(1,3),$ and in which direc$-$

tion does it occur?