Answer:
Velocity of center of mass must be ZERO
Explanation:
As we know that two cars are moving on friction-less surface
So here we can say that on the system of cars there is no external force along x direction
So we can say that
so here if net force is zero
since finally the two cars comes to rest so here final velocity of center of mass of two cars will be zero and hence it will be always zero as there is no force on the system of two cars