A block of mass $m=0.1 \;kg$ is connected to a spring of unknown spring constant $k$. It is compressed to a distance $x$ from its equilibrium position and released from rest. After approaching half the distance$(\frac{x}{2})$ from equilibrium position, it hits another block and comes to rest momentarily, while the other block moves with a velocity $3 \;ms^{-1}$ . The total initial energy of the spring is :
