initial velocity

$v^{2}=\omega^{2}\left(A^{2}-\left(\frac{2 A}{3}\right)^{2}\right)$                $...(1)$

Where $A$ is initial amplitude $\&\, \omega$ is angular frequency.

Final velocity

$(3 v)^{2}=\omega^{2}\left(A^{\prime 2}-\left(\frac{2 A}{3}\right)^{2}\right)$                $...(2)$

From equation $\&$ equation $( 2)$

$\frac{1}{9}=\frac{A^{2}-\frac{4 A^{2}}{9}}{A^{2}-\frac{4 A^{2}}{9}}$

$A^{\prime}=\frac{7 A}{3}$