Assuming I've had sufficient coffee this morning...
nRT = (P + (an^2)/V^2)*(Vnb), find the partial V respect to T
Rescaling to lump all the constants into a minimal set [for the purpose of the partial w.r.to T, I presume that everything but V and T are constants]:
T = (b + 1/V^2)*(Vc). Differentiate both side w.r.to T to get, using dV as shorthand for "partial of V w.r.to T":
1 = (b  2*dV/V^3)*(Vc) + (b + 1/V^2)*(dVc), now just rearrange to isolate dV.
You can do it with all the original constants carried through.
