So in my homework I was able to get this-
$\left(\frac{\partial U}{\partial V}\right)_T = T(\frac{\partial P}{\partial T})_V - P$
but now I'm trying to plug in $P = \frac{nRT}{V-nb}-a(\frac{n}{V})^2$ in the above equation. I'm struggling here, I got zero for $\left(\frac{\partial U}{\partial V}\right)_T$ but I'm then asked to evaluate $\left(\frac{\partial U}{\partial V}\right)_T$ in an integral from infinity to $V$ with respect to $V$ (i.e. $dV$).