Two particles, of masses m and 3m, are moving toward each other along the x-axix with the same initial speeds of 9.81 m/s. Mass m is travelling to the left, and mass 3m is traveling to the right. They undergo a head-on elastic collision and each rebounds along the same line as it approached. Find the final speed of the heavier particle. Not sure how to go about it, if someone could please help!