A 1kg cart, rolling at 6m/s, collides with and sticks to an identical cart that's initially at rest. So, after colliding, the carts roll together as...

4. A 1kg cart, rolling at 6m/s, collides with and sticks to an identical cart that’s initially at rest. So, after colliding, the carts roll together as a single, 2kg unit. How fast does the pair of carts roll? (Suppose friction is very small, compared to other forces in the problem.)