$M =$ mass, $\rho  =$ density, $A = $Area of cross section 

$V = $ volume, $l =$ length, $\Delta l$ = change in length 

Then mass per unit length $m = \frac{M}{l} = \frac{{Al\rho }}{l} = A\rho $ 

And Young’s modules of elasticity

$y = \frac{{T/A}}{{\Delta l/l}}$ 

==> $T = \frac{{Y\Delta lA}}{l}$. 

Hence lowest frequency of vibration $n = \frac{1}{{2l}}\sqrt {\frac{T}{m}} $

$ = \frac{1}{{2l}}\sqrt {\frac{{y\left( {\frac{{\Delta l}}{l}} \right)A}}{{A\rho }}} = \frac{1}{{2l}}\sqrt {\frac{{y\Delta l}}{{l\rho }}} $ 

==> $n = \frac{1}{{2 \times 1}}\sqrt {\frac{{9 \times {{10}^{10}} \times 4.9 \times {{10}^{ - 4}}}}{{1 \times 9 \times {{10}^3}}}} = 35Hz$