How far must the point charges q1 = +7.22 µc and q2 = -32.5 µc be separated for the electric potential energy of the system to be -117 j? cm?

The formula that we wil be using is the formula for two charges q and Q separated by a distance r which is: U = kQq/r 
Where:
U = electric potential,
Q and q = point charges, and
r = the distance between the point charges.
 So we are looking for 'r', rearranging the formula would give us:
U*r = kQq
r = kQq/U -> this will be the formula that we will be using.
r = (9 x 10^9)*(7.22 x 10^-6)*(-32.5 x 10^-6) / -117 
r = .01805 m