Trains colliding and latching together

[mrshappy0]

Homework Statement

A train car with mass m1 = 550 kg is moving to the right with a speed of v1 = 7.2 m/s and collides with a second train car. The two cars latch together during the collision and then move off to the right at vf = 4.7 m/s.2)What is the mass of the second train car?

Homework Equations

momentum before = momentum after.

kinetic energy before does not equal kinetic after.

The Attempt at a Solution

okay, I assumed the second car is not moving and solved for v final using this:

m1v1+m2v2=(m1+m2)v final.

My answer was wrong. Any ideas. My guess is that I shouldn't assume the second train car isn't moving but I don't know another way to solve it in that case.

 

[e4canadian]

How do u know that your answer is incorrect i attempted your problem your equation m1v1+m2v2=(m1+m2)vf appears correct to me it gives 550*7.2+0=(550+m2)4.7 and I got an answer of 292.55 kg.