A railroad car that weighs 20,000 lb is traveling eastward with a

Question

velocity whose magnitude is 5fps. A second car, on the same track, that
weighs 40,000 lbs also traveling in an easterly direction with a
velocity of 7.8fps. When the cars collided, they became coupled together
and then both cars moved with the same velocity. If friction between the
cars and the rails is neglected, what were the magnitude and the
direction of the velocity of the car after the collision?

Answer 1

Momentum before = m1 v1 + m 2 v2

Momentum after = (m1+m2) v

then
momentum after = momentum before.