At the same instant that a motionless 9.6 kg ball is dropped from 27 m above the Earth's surface, a second ball, with a mass of 4.5 kg, is thrown straight upward from the Earth's surface with initial velocity of 21.1 m/s. They move along nearby lines and pass without colliding. At the end of 2.1 s, what is the velocity of the center of mass of the two-ball system?