Energy $=90 \,J \quad$ Amplitude $=6 \,cm$

Maximum energy $=\frac{1}{2} m A^2 \omega^2=90$

$\text { or } m \omega^2=\frac{180}{36 \times 10^{-4}}$

$\therefore m \omega^2=30 \times 10^2$

When particle is stopped the point where it is stopped is the new amplitude but angular velocity will remain same.

$E=\frac{1}{2} m A_2^2 \omega^2$

$\text { or } E=3000 A_2^2$

$A_2=4 \times 10^{-2}$

$A_2=3000$