2 objects called A and B are weighted.3KG and 2KG.A has 2m/s velocity and B has 6m/s before the collision and they collide face to face.after the collision ,object A moves to opposite direction with a speed of 4m/s.What is the velocity of B after the collision?

answer has to be 15m/s.But I never get it

here how i did it

put m1u1+m2u2=m1v2+m1v2 to the right

3kg * 2m/s + 2kg *( -6m/s) = 3kg * (-4m/s)+2v

=6-12=-12+2v

v=3m/s not 15