Potential Difference Question

ATeam232

1. The problem statement, all variables and given/known data
Point charges of equal magnitudes (50 nC) and opposite signs are placed on diagonally opposite corners of a 60 cm 90 cm rectangle. Point A is the unoccupied corner nearest the positive charge, and point B is the other unoccupied corner. Determine the potential difference VB - VA.

2. Relevant equations

v=Kq/r

3. The attempt at a solution

2 * SQRRT( (kq/r1)^2+(kq/r2)^2) yielded the wrong answer...however I am not quite sure what to use as my radii. Do I use the distance from the corners to the center? Or do I use the entire distance from corner to corner? I am fairly certain that my equation is correct. Any help would be appreciated.