Suppose z = f(x, y), where x = g(s, t), y = h(s, t), g(1, 2) = 3, g_s(1, 2) = −1, g_t(1, 2) = 4, h(1, 2) = 6, h_s(1, 2) = −5, h_t(1, 2) = 10, f_x(3, 6) = 7, and f_y(3, 6) = 8. Find ∂z/∂s and ∂z/∂t when s = 1 and t = 2.