Find the potential between two non-conducting parallel planes How would I solve the following?

Two uniformly charged, infinite, nonconducting planes are
  parallel to a yz plane and positioned at x  50 cm and x     50
  cm. The charge densities on the planes are 50nC/m2 and  25
  nC/m2, respectively. What is the magnitude of the potential difference
  between the origin and the point on the x axis at x     80 cm?
  (Hint: Use Gauss’ law.)
  •••

If I used cylindrical Gaussian surfaces, would I need to use three of them? If I did use three Gaussian cylindrical surfaces how would i find the magnitude of potential difference?

 A: Since the plates are infinite and parallel, you can deduce something about the electric field. In which direction do the electric field lines have to be?

 They must be perpendicular to the surfaces. They cannot be at any angle because the infinite parallel plates do not have any direction, other than perpendicular, that would be special. So the only non-zero component can be $E_x$.

Now that you know the direction of the field lines, you can simplify the application of Gauss' law. What shape would make sense as an enclosed volume?

 As the field lines are perpendicular to the plates, it make most sense to use a box. The field lines will only pierce the box on the faces parallel to the $y$-$z$ plane.

Since the plates are infinite of size, what does that mean for the field. Does it have a different strength at different $y$-$z$ coordinates?

 No, no point on the plane is special, it must be the same on each point a $y$-$z$ plane with fixed $x$.

What does that imply for the application of Gauss' law?

 The integration over the electric flux becomes a simple multiplication: $E \cdot A$ where $A$ is the area of the face that the flux goes through.

Now we need to compute the field strength between the plates and outside the plates. You must choose the volume in a clever way in order to say something about the field strength at this point. You may put part of the surface into the plate. What can you say about the electric field inside a conductor?

 There cannot be any electric field inside the conductor.

With this you should be able to compute the electric field between the plates and on either side of the two infinite plates.
