Their electrical potential energy is 22,500 J.
The electric potential energy associated with two charges,
As you can see, this is simply a plug-and-play problem in which you use the given values to determine
SInce one spere is 75 mm from point P and the other is 25 mm from the same point P, in opposite directions, it follows that the distance between them is equal to
This means that their electric potential energy is