Answer: = 5 m/s
Explanation: For collisions, use linear momentum, whose formula is: P = m.v and knowing the law of conservation states that momentum of a closed system is constant in time, i.e. initial momentum equals final momentum:
Suppose the referential for velocity is positive to the right:
The negative sign, in this case, indicates cart A is going to the left.
The magnitude of velocity of cart A is 5 m/s