posted by K .
Two adjacent corners of a square have charges q1 = +1.5 x 10-9 C and q2 = +4.0 x 10-9 C. The length of a side of the square L = 0.25 m. Find the EPE of a charge q3= -6.0 x 10-9 C placed at the 2 remaining corners.
I assume that EPE stands for Electrical Potential Energy. Are you asking for the total EPE of the complete configuration of 4 charges? It is confusing when you talk of the EPE of A (one) charge q3 placed at two corners.
The way you compute the EPE of one charge Q3, relative to a an energy of zero when the charge is at infinite dstance, is to add up the k Qi Q3/ri terms due to the other charges Qi at distance ri. k is Boltzmann's cosntant.