A softball of mass 0.220 kg that is moving with a speed of 6.7 m/s collides head-on and elastically with another ball initially at rest. Afterward it is found that the incoming ball has bounced backward with a speed of 4.2 m/s. (Assume the positive direction is forward.) (a) Calculate the velocity of the target ball after the collision. (b) Calculate the mass of the target ball. I know I have to use conservation of momentum along with conservation of energy, but I don't know how to set them up or combine the 2 equations.