Mean square radius uniformed charged sphere

[malawi_glenn]

Homework Statement

Hi i am going to proove that the mean square radius for a uniformed charged sphere is:

For a sphere with radius R.

Homework Equations

So how do i do this? I have no idea where is should beging and so on.

The Attempt at a Solution

I thougth like this:

But that makes no sense, i just know what the solotion is so.. help