Force between parallel plates in a capacitor

[asi123]

Homework Statement

Hey guys.
So, I got this problem.
According to their hint I'm suppose to calculate the work of moving one of the plates and from their, get to the force.
I don't have a clue how to do that. I can't find a single formula that can help with that.
Can I please have some help...?

Thanks.

Homework Equations

The Attempt at a Solution

 

[LowlyPion]

Here. You can watch the first part of this lecture:

https://www.youtube.com/watch?v=7NUbsQt-G9U