$F_B=m g \Rightarrow 3 \rho A y\,g=\rho A\left(60 \times 10^{-2}\right)\,g$

$y=20\,cm$

$\Rightarrow \text { Maximum amplitude }=20\,cm$

Restoring force when it is slightly depressed by an amount of $x$.

$F =-(\Delta V \sigma g )=-( A \sigma g ) x$

$T =2 \pi \sqrt{\frac{ m }{ A \sigma g }}=2 \pi \sqrt{\frac{\rho A h}{3 A \rho g}}=2 \pi \sqrt{\frac{ h }{3 g}}$

$=2 \pi \sqrt{\frac{60 \times 10^{-2}}{3 \times 9.8}}=\frac{2 \pi}{7}$