Electric Potential -- work due to a non-conducting ring

[m305]

Homework Statement

A charge of q1=-9.8 nC is uniformly distributed around a thin plastic ring lying in a yz plane with the ring center at the origin. A q2= -8.0 pC point charge is located on the x axis at x = 3.4 m. For a ring radius of 1.2 m, how much work must an external force do on the point charge to move it to the origin?

Homework Equations

U=W= (q1q2)/(4πεr)

The Attempt at a Solution

it says in the solution that W=ΔU= ( q1q2)/(4πε) * ( 1/r - 1/ √(r^2 + x^2) ) ; can someone please explain where did we get the ( 1/r - 1/ √(r^2 + x^2) ) from?
Thank you !

 

[vela]

You got it from the solutions...nyuck nyuck nyuck

What do r and x represent?