Find the mass and velocity of the second particle.

A proton of (mass 1u) moving at 6.9x10^6 m/s collides elastically and head-on with a second particle moving in opposite direction at 2.80x10^6 m/s. After the collision, the proton is moving opposite to its initial direction at 8.62x106 m/s.Find the mass and velocity of the second particle.

For elastic collisions you need to simultaneously conserve linear momentum and energy.
Do not lose the minus signs associated with "head-on" and "opposite to its initial direction" !!!

For particles a and b at times 1 and 2:
KEa1 + KEb1 = KEa2 + KEb2
MaVa1 + MbVb1 = MaVa2 + MbVb2