This question concerns a collision of two identical particles, A and B, each of mass m, which constitute an isolated system that is observed from an inertial frame. Assume that the particles, when sufficiently close, interact only through a potential energy function. At an instant long before the collision (when the distance between A and B is sufficiently large for the potential energy to be neglected) A has position (d, 0, -f) and velocity (0, 0, V) while B has position (-d, 0, f) and velocity (0, 0, -V), where you should assume f and V and d are positive constants.
The solutions deals with the collision while the particles interacts at large distances by conserving momentum and energy. This also discuss the position during the interaction.