Conservation of linear momentum!

Before collision momentum is 21m-9m kg.m/s. After collision the momentum is 10m v kg.m/s, so:

30m = 10m v

so v=3 m/s in the same direction as the original motion of the heavier truck

The impulse on B is equal to its change in momentumii)find in terms ofmthe magnitude of the impulse A ON B in the collision

You should be able to do this, you know the initial velocity, assume a constant breaking force.Immediately after the collision the truck experiences a braking force which brings them to rest in 24metres.

ii)fidn the magnitude of the braking force.

RonL