Problem

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 \times 10^{-8} \text{ J}

. When the second charge is moved to point

b

, the electric force on the charge does

4.2 \times 10^{-8} \text{ J}

of work. What is the electric potential energy of the pair of charges when the second charge is at point

b