Express $321_7-321_5$ in base 10.
We subtract  $$
\begin{array}{cccccc}
&3\cdot 7^2 &+& 2 \cdot 7 &+& 1 \\
-&(3\cdot 5^2 &+& 2 \cdot 5 &+& 1) \\ \hline
&3\cdot(7^2-5^2)&+&2\cdot 2 & &
\end{array}
$$Evaluating $ 3\cdot(7^2-5^2)+2\cdot 2$, we get $\boxed{76}$.