A 0.10-kg object with a velocity of 0.20 m/s in the +x direction makes a head-on elastic collision with a 0.15 kg object initially at rest. What is the final velocity of the 0.10-kg object after collision?

Newton's second law in terms of momentum:  The rate of change of momentum of an object is equal to the net force applied to it.
The law of conservation of momentum: The total momentum of an isolated system of objects remains constant.

Newton's second law for a system of particles: The sum of all forces acting on the system is equal to the total mass of the system times the acceleration of its center of mass.