Potential due to two infinite parallel planes

[Lisa...]

Consider two infinite parallel planes of charge, one in the yz plane and the other at distance x = a. (a) Find
the potential everywhere in space when V = 0 at x = 0 if the planes carry equal positive charge densities + σ. (b)
Repeat the problem with charge densities equal and opposite, and the charge in the yz plane positive.

I haven't had any problems with part a. As for part b, E=0 if x > a (the field lines of both planes cancel) , so I figured the potential needs to be 0 too, but the answer should be V= $$\frac{- \sigma a}{\epsilon_0}.$$ Why is that?

EDIT: I just figured it out by myself :)

 

[member 553083]

The potential difference between the planes is V = \frac{- \sigma a}{\epsilon_0}, so the potential at x=0 must be - \frac{ \sigma a}{\epsilon_0}.