Question

Let w = −3 xy + 2 x + 9 y, x = r + s + t, y = r + s, and z = s + t. Find the partial derivatives of w with respect to r, s and t at the point r =−5, s =−1, t = 4.
∂w/∂r = ???
∂w/∂s = ???
∂w/∂t = ???