Given: Mass of the block m=500g, initial velocity of the block u=12ms−1, and final velocity of the block v=2u=6ms−1.
As non-conservative forces are not acting on the system we can conserve energy.
Thus, loss in kinetic energy of block = Gain in potential energy of spring
21mu2−21mv2=21kx2
21×0.5×122−21×0.5×62=21k×0.32
k=600Nm−1