Let the CSA of the wire be A.​ Stress = Area  Force ​=AMg​ Strain =Ll​ Volume =AL​ We need to calculate the elastic potential energy stored in the wire which is given to be equal to 21​× Stress × Strain × Volume.​ Elastic potential energy =21​× Stress × Strain × Volume =21​×AMg​×Ll​×AL=21​Mgl​ The other 21​Mgl is converted into kinetic energy of the mass.When the mass leaves its initial point on the spring, it acquires a velocity as it moves down. The velocity reaches its maximum at the end point. The spring oscillates. Finally, when the kinetic energy is dissipated into heat, the spring comes to rest.