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, -u² + v + 4), find gu (2, 1) step by step.