a
$\mathrm{T}=\mathrm{mg}$
$\sigma=\frac{\mathrm{T}}{\mathrm{A}}=\frac{\mathrm{mg}}{\mathrm{A}}$
$\frac{(\sigma \mathrm{A} \ell) \mathrm{g}}{\mathrm{A}}$
$\Rightarrow \ell=\frac{\sigma}{\rho \mathrm{g}}=\frac{1.2 \times 10^8 \times 3}{6 \times 10^4 \times 10}=600$
