Velocity after the collision

[physicsgurl12]

Homework Statement

a 480kg car moving at 14.4m/s hits a another car from behind moving at 13.4m/s in the same direction. if the second car has a mass of 570kg and a new speed of 17.9m/s what is the velocity of the first car after the collision??

Homework Equations

m1*v1=m2*v2
doubt that's it

The Attempt at a Solution

i know its 8.94m/s but why??

 

[rl.bhat]

You have to use the law of conservation of linear momentum. According to this law, the total momentum before collision is equal to the total momentum after collision..
So m1u1 + m2u2 = m1v1 + m2v2.