posted by bird .
A proton (mass=1.67*10^-27 kg) moves with a velocity of 6.00*10^6 m/sec. Upon colliding with a stationary particle of unknown mass, the proton rebounds upon its own path with a velocity of 4.00*10^6 m/sec. The collision sends the unknown particle forward with a velocety of 2.00*10^6 m/sec. What is the mass of the unknown particle.