Electric Potential Difference?

Question

One particle has a mass of 3.00 x10^-3 kg and a charge of +8.40 µC. A second particle has a mass of 6.00 x10^-3 kg and the same charge. The two particles are initially held in place and then released. The particles fly apart, and when the separation between them is 0.100 m, the speed of the 3.00x^10-3 kg particle is 115 m/s. Find the initial separation between the particles.

Answer 1

Use conservation of momentum to find velocity of second particle
Then calculate total KE of both and set equal to change in potential energy from the initial separation x to .1 m and solve for x.