An object of mass 0.5 kg is executing simple harmonic motion.

$\mathrm{T}=2 \pi \sqrt{\frac{\mathrm{m}}{\mathrm{k}}}$

$0.2=2 \pi \sqrt{\frac{0.5}{k}}$

$\mathrm{k}=50 \pi^{2}$

$\approx 500$

$x=A \sin (\omega t+\phi)$

$=5 \mathrm{~cm} \sin \left(\frac{\omega \mathrm{T}}{4}+0\right)$

$=5 \mathrm{~cm} \sin \left(\frac{\pi}{2}\right)$

$=5 \mathrm{~cm}$

$\mathrm{PE}=\frac{1}{2} \mathrm{kx}^{2}$

$=\frac{1}{2}(500)\left(\frac{5}{100}\right)^{2}$

$=0.6255$