Understanding Electric Force: Calculating Change in Potential Energy

[soljaragz]

Here is the question that I have no idea what is happening

"What is the change in potential energy of a particle of charge +q that is brougt from a distance of 3r to a distance of 2r by a particle of charge -q? "

Here's what I tried

-W = deltaU
W = qEd
W = (kq1q2/r^2) d
d= 3r - 2r = r

substituting I get
W = kq1q2/r
so deltaU = -kq1q2/r.....im correct up to here, but its not the final answer according to the book,...the explanation then shows some weird things that I don't understand.

but isn't the question asking for change in potential
so shouldn't deltaU (-kq1q2/r) be the answer?

 

[Doc Al]

Hint: The potential energy of two charges separated by a distance r is given by kq1q2/r (where the zero level is taken to be at r = infinity).

 

[soljaragz]

[nvm i get it now]
I have another problem though