4. If f(x, y) is a differentiable function of x and y, fx (1, 1) = 12, fx (2, 1) = 2, fy(1, 1) = 3 and fy(2, 1) = 5. Let g(u,v) = f(u v,u +v+4), find gu (2, 1) step by step. (5 points)