A railroad car of mass M moving at a speed v1 collides and couples with two coupled railroad cars, each of the same mass M and moving in the same direction at a speed v2.

(a) What is the speed vf of the three coupled cars after the collision in terms of v1 and v2? (Use M, v_1 for v1, and v_2 for v2 as appropriate.)

a) (M*v_1+M*v_2+M*v_2)/(3M)

( How much kinetic energy is lost in the collision? Answer in terms of M, v1, and v2. (Use M, v_1 for v1, and v_2 for v2 as appropriate.)

I don't get this I had this for the answer. 1/2*M*[-2/3*v_1^2 +4/3*v_1*v_2-2/3v_2^2]
but it is wrong...

thanks