A point charge `q_1` is held stationary at the origin. A second charge `q_2` is placed at a point a, and the electric potential energy of the pair of charges is `-6.4 xx 10^-8 J`. When the second charge is moved to point b, the electric force on the charge does `4.2 xx 10^-8 J` of work. What is the electric potential energy of the pair of charges when the second charge is at point b?
