Two balls collide in a head-on elastic collision and rebound in opposite directions. One ball has velocity 1.2 m/s before the collision, and −2.3 m/s after. The other ball has a mass of 1.1 kg and a velocity of −4.2 m/s before the collision. (a) What is the mass of the first ball? (b) What is the velocity of the second ball after the collision?