Pchem thermodynamics help?

Hey everybody, I have been working on this problem for a good while now and am getting nowhere so i thought id try my luck here.

how can i derive equations for the change in Gibbs free energy and Helmholtz free energy for the isothermal expansion of a van der waals gas from v(1) to v(2)?

I know that at constant temperature dG=VdP and then to take the integral of v where v is the volume in terms of the van der waals equation. but i am not sure how to apply vanderwaals equation and integration from there or if i am even going in the right direction?

thanks