(b) Given :$l = 4.9 \times 10^{- 4}m$
From the formula
$Y = \frac{stress}{strain} = \frac{F/\pi r^{2}}{l/L}$
Or
$\frac{F}{\pi r^{2}} = Y\frac{l}{L}$
=$9 \times 10^{10} \times \frac{{{\ \ \ \ 4.9 \times 10}^{10}}^{-}}{4}\ \ $
=44.1$\times 10^{6}$
$V = \frac{1}{2L}\sqrt{\left( \frac{T}{m} \right)}$
$= \frac{1}{2L}\sqrt{\left( \frac{F}{\pi r^{2}d} \right)} = \frac{1}{2}\sqrt{\left( \frac{F/\pi r^{2}}{d} \right)}\ $
$= \frac{1}{2} \times \sqrt{\left( \frac{44.1 \times 10^{6}}{9 \times 10^{3}} \right)}$
$= \frac{1}{2}\sqrt{\left( 4.9 \times 10^{3} \right)} = \frac{1}{2}\sqrt{\left( 49 \times 10^{2} \right)}$
$= \frac{7 \times 10}{2} = 35\ Hz$