A 2.7 kg cart is rolling across a frictionless, horizontal track toward a 6.4 kg cart that is held initially at rest. The carts are loaded with strong magnets that cause them to attract one another. Thus, the speed of each cart increases. At a certain instant before the carts collide, the first cart's velocity is 4.1 m/s, and the second cart's velocity is -1.9 m/s. What was the velocity of the first cart when the second cart was still at rest