Elastic Collisions in One Dimension (Problem 29)

Professional Application Two piloted satellites approach one another at a relative speed of 0.250 m/s, intending to dock. The first has a mass of $4.00\times {10}^3\text{ kg}$, and the second a mass of $7.50\times {10}^3\text{ kg}$. If the two satellites collide elastically rather than dock, what is their final relative velocity?