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.
\dfrac∂w∂r = ???.
\dfrac∂w∂s = ???.
\dfrac∂w∂t = ???.