
At first the block is pulled to a distance x0=10cm(extreme position).
Potential energy of block is Ui=21kx02 and kinetic energy os block, Ki=0.
Now, when the block is at distance 5cm.

Potential energy is Uf=21k(2x0)2 and kinetic energy is Kf=0.25J
Total energy of block will be conserved, so
21kx02+0=21k4x02+0.25
⇒21kx0243=41
⇒21k1003=1⇒k=3200Nm−1
=67Nm−1