Potential energy of a proton at the midpoint of a capacitor

[csimon1]

Homework Statement

I have attached the image. The question is --

What is the potential energy of a proton at the midpoint of the capacitor?

I know this is an extremely easy problem, but I can't seem to get the formula right...

Homework Equations

I tried U = qV, using 1.6*10^-19 * 150V (because it is in the middle). What am I doing wrong? Is this not the right formula?

The Attempt at a Solution

 

[fluidistic]

Hi and welcome to PF!
I know a hard way to solve the question. I'm not sure it is of help in your case, but in case you've taken calculus 3 then it's ok.
$$\phi (r)= \int _{\infty}^r \vec E \cdot d \vec l$$. Thus the almost only difficulty would be to calculate the E field between the plates of the capacitor.

There might be of course a much faster way to solve it. So if anyone can help, welcome!