$ When\, additional \,mass \mathrm\,{M}$ is added then
$\mathrm{T}_{\mathrm{M}}=2 \pi \sqrt{\frac{\ell+\Delta \ell}{\mathrm{g}}}$
$T_{\frac{M}{T}}=\sqrt{\frac{\ell+\Delta \ell}{\ell}}$ or $\left(\frac{T_{M}}{T}\right)^{2}=\frac{\ell+\Delta \ell}{\ell}$
or, $\left(\frac{T_{M}}{T}\right)^{2}=1+\frac{M g}{A y}\left[\because \Delta \ell=\frac{M g \ell}{A y}\right]$
$\therefore \frac{1}{y}=\left[\left(\frac{T_{M}}{T}\right)^{2}-1\right] \frac{A}{M g}$
