7. (20 pts) Consider the surface given by z wy – 4xy + 3x – 2y. (a) Find the equation of the tangent plane to this surface at the point where (x,y) – (1,3). (b) Find the gradient f at the point where (x,y) = (1,3). (c) Find the directional derivative DaS(1,3) where it is the unit vector in the direction of (1, -2). such that the directional derivative Daf(1.3) is a maximum (d) Find a unit vector in the direction of u