We were told that two objects became stuck together hence we have to use the principle of conservation of momentum to obtain the final velocities of the carts.
The principle of conservation of momentum lets us know that the momentum before collision is equal to the momentum after collision. As such we can write; m1u1 + m2u2 = m1v1 + m2v2.
We can use this thus principle to obtain the final speeds of the carts since the two objects that collided became stuck together.
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