A 9500 kg boxcar traveling at 19 m/s strikes a stationary second car. The two stick together and move off with a speed of 10 m/s. What is the mass of the second car?

Answer 1

Apply the law of conservation of linear momentum. Let the second (struck) car's mass be M2, and the first car's mass be M1.

M1*19 = (M1 + M2)*10

Solve for M2
9 M1 = 10 M2

You finish it.