Dividing $(ii)$ by $(i)$ we get ${\left( {\frac{7}{6}} \right)^2} = \frac{{m + m'}}{m} = \frac{{49}}{{36}}$
$\frac{{m + m'}}{m} - 1 = \frac{{49}}{{36}} - 1 \Rightarrow \frac{{m'}}{m} = \frac{{13}}{{36}}$ $⇒$ $m' = \frac{{13m}}{{36}}$
Also $\frac{k}{m} = \frac{{4{\pi ^2}}}{{{{(0.6)}^2}}}$
Desired extension $ = \frac{{m'g}}{k}$$ = \frac{{13}}{{36}} \times \frac{{mg}}{k}$
$ = \frac{{13}}{{36}} \times 10 \times \frac{{0.36}}{{4{\pi ^2}}} \approx 3.5\;cm$
(Young's modulus of material of track is $10^{11} \,{Nm}^{-2}$ ))