Two blocks of masses M and 4M 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, as seen in the figure below. A cord initially holding the blocks together is burned; after this the block of mass 4M moves to the right with a speed of v1=1.58 m/s. What is the velocity of the block of mass M? Calculate the original elastic energy in the spring if M = 0.365 kg.