Find a function f(x, y, z) such that V f is the constant vector (9, 9,4). (Use symbolic notation and fractions where needed. Use C for the constant of integration.) ey ] Find a unit vector n that is normal to the surface 2z x* y4 = l at P = (1,2, 3) that points in the direction of the xy-plane (in other words, if you travel in the direction of n, you will eventually cross the xy-plane). (Use symbolic notation and fractions where needed.) Find the directional derivative of f(x,y.z) = xy + z at the point P = (2,7,-2) in the direction pointing to the origin. (Give an exact answer. Use symbolic notation and fractions where needed.) DFE)P) =