A particle A of mass 2 kg is moving along a straight horizontal line with speed 12 m/s. Another particle B of mass m kg is moving along the same straight line in the opposite direction to A, with speed 8 m/s. The particle collide. The direction of motion of A is unchanged by the collision. Immediately after the collision, A is moving with speed 3 m/s and B is moving with speed 4 m/s. Find the value of m.