Calculating Change in Electric Potential Energy with Multiple Charges

[watdawaq]

Homework Statement

A 33uC point charge is placed 36cm from an identical 33uC charge. A -2.5uC charge is moved from point a to point b in the figure:
http://session.masteringphysics.com/problemAsset/1096949/4/GIANCOLI.ch17.p72.jpg

Homework Equations

U=kq1q2/r

The Attempt at a Solution

I found Ui by using above equation for each r from the -2.5uC charge and found the difference.

Repeated for Uf when charge is moved from a->b.

Uf-Ui = 1.74J, but MP says this is incorrect. What am I missing?

 

[Doc Al]

Please show the details of your calculation.