One particle has a mass of 4.00 10-3 kg and a charge of +9 µC. A second particle has a mass of 7.00 10-3 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 0.091 m, the speed of the 4.00 10-3-kg particle is 133 m/s. Find the initial separation between the particles.