Mass = m, Velocity = v

Then,

Initial K.E. = 1/2 mv²

Final K.E. after object comes to rest

Final K.E. = 0

‍

Hence,

From the work-energy theorem

Work done = Change in kinetic energy

                  = Final K.E - Initial K.E

                  = 0 - 1/2 mv²

                  = - 1/2 mv²