Let z = f(x, y), where f is differentiable, and x = g(t) and y=h(t). Suppose that g(3) = 2, h(3) = 7, g'(3) = 5, h'(3) = -4, fx(2, 7) = 6 and fy(2, 7) = -8. Find dz/dt when t = 3.