Two blocks of masses 8 kg and 21 kg are placed on a horizontal, frictionless surface. A light spring is attached to one of them, and the blocks are pushed together with the spring between them. A cord holding them together is burned, after which the block of mass 21 kg moves to the right with a speed of 13 m/s. What is the velocity of the other mass in m/s? Answer in units of m/s.