$x=A \sin (\omega t+\phi)$
$\text { at given time }$
$\Rightarrow \omega t+\phi=30^{\circ}$
$\Rightarrow x=40 \times \frac{\sqrt{3}}{2} \Rightarrow 20 \sqrt{3}\,cm$
$\Rightarrow A=40\,cm$
$\Rightarrow K . E=\frac{1}{2} k\left(A^2-x^2\right)=200$
$200=\frac{1}{2} k\left(\frac{1600-1200}{100 \times 100}\right)$
$400 \times 100 \times 100=k \times 400$
$k=10^4$
$x=4$