A Physics Problem involving collisions !

Three objects A,B and C are kept in a straight line on a frictionless horizontal surface.They have masses m,2m and m respectively.The object A moves towards B with a speed of 9 m/s and makes an elastic collision with it.Thereafter B makes completely inelastic collision with C.Find the final speed in meter/sec of C.All collisions occur in the same straight line.

let the velocity of A and B after collision be = V1 and V2 respectively.
Then,conservation of momentum gives , m*9 + 0 = m*V1 + 2m(V2)
and V2 - V1 = e(u1 - u2) = 9 (u1 is the initial velociyt of A and U2 is the initial velocity of B and e is the coefficient of restitution.)
Solving these two gives, V1= -1 m/s and V2 = 6 m/s
Again,conservation of momentum gives ( for B and C) 3m* V = 2m*6 ( V is the required velocity)
So... V = 4 m/s
Thanks.