DUE 45 minutes !!!

Two particles, of masses m and 4m, are moving toward each other along the x−axis with the same initial speeds of 1.87 m/s. Mass m is traveling to the left, and mass 4m is traveling to the right. They undergo a headon elastic collision, and each rebounds along the same line as it approached. Find the final speed of the heavier particle. Answer in units of m/s.