Ok, so based on my boundary conditions, I think I have to expand f(x,y) as a double Fourier cosine expansion, so:
f(x,y)= 1/2Ʃ(C2cos(n∏x/a) + C3cos(n∏y/b)) + ƩƩC4cos(m∏x/a)cos(n∏y/b)
Also, I can let F(x,y)=u(x,y)=X(x)Y(y)
Plugging that into the original Poisson equation I end up with...