(where Q0 through Q3 are the points of the quad. D could be rewritten as D = -(A*x + B*y + C*z) where x,y,z are the components of Q0, if that makes more sense to you.)
so expanding it out a bit:
[indent=1]Py = -(Nx * Px + Nz * Pz - (N dot Q0)) / Ny
I'm sure a google search or two could explain the details better than I have, but hopefully this is enough info to get you going.