As the question is incomplete I can provide you with the concept when a body two body collides in a perfectly inelastic collision then the coefficient of restitution that is the ratio of velocity of separation to the velocity of approach becomes zero this the velocity of separation becomes zero in that case the two colliding bodies will move together just after the collision so if there is a angle of 180 degree between their motion and are of same mass and same velocity then after the collision they will become at rest apart from that you can apply the conservation of linear momentum in any collision but the conservation of energy will not be applicable in case of inelastic collisions because there is some energy loss occurs.

Hello

If you want to know about the speed of the particle, I think it will become half the initial speed.
If it is an incomplete question, kindly complete the question.
Good Luck!