Electric potential due to 3 charge

[walkerob08]

Homework Statement

There are three charges q1, q2 and q3 situated at corners of a rectangle of sides a = 15.0 cm and b = 8.0 cm.

For q1 = 4.90 μC, q2 = -4.90 μC, and q3 = 1.50 μC

a) find the electric potential at the center of the rectangle.

b) Continuing with the system above, how much of the electric energy of the system would be expended in moving q3 to infinity while q1 and q2 remain in their positions?

Homework Equations

V = kq/r

EPE = qV

The Attempt at a Solution

So I started this by thinking of bringing these point charges in from infinity one at at time. I brougt q1 in first and found the electric potential it would establish, taking effect when I brought the other in. But I really have no idea where to go from here. Thanks in advance for the help!

 

[SammyS]

It's not asking for the potential energy of the system. It's asking for the electric potential at a specific point, namely the center of the rectangle.