When a sound wave travels from one medium to another medium its frequency remains the same

$\because \quad$ Frequency, $v=\frac{v}{\lambda}$

since $v$ remains the same in both the medium

$\Rightarrow \quad \frac{v_{\text {air }}}{\lambda_{\text {air }}}=\frac{\nu_{\text {brass }}}{\lambda_{\text {brass }}}$

$\lambda_{\text {brass }}=\lambda_{\text {air }} \times \frac{V_{\text {brass }}}{V_{\text {air }}}=\lambda_{\text {air }} \times \frac{3500}{350}=10 \lambda_{\text {air }}$