Use the Chain Rule to find the indicated partial derivatives.
$$
\begin{array}{l}
z=x^{4}+x^{2} y, \quad x=s+2 t-u, \quad y=s t u^{2} \\
\frac{\partial z}{\partial s}, \frac{\partial z}{\partial t}, \frac{\partial z}{\partial u} \quad \text { when } s=4, t=2, u=1
\end{array}
$$