Physics10
posted by Stanley .
Two identical gliders slide toward each other on an air track. One moves at 1m/s and the other at 2m/s. they collide and stick. the combined mass moves at
A. 1.5m/s
B. 1/3m/s
C. 1/6m/s
D. 3/4m/s
E. 1/2m/s
which of this options is the right answer?

The option that preserves total momentum is the correct one.
Note that the final mass is 2M, and that the initial individual momenta have opposite signs.
(2m/s)*M (1m/s)*M = 2M*Vfinal
Solve for Vfinal 
could you help me and solve it please. I tried to solve it but I couldn't get the right answer.