a
$f_{c}=f_{0}$
$\frac{3 V_{C}}{4 L}=\frac{2 V_{0}}{2 L^{\prime}}$
$\frac{3 V_{C}}{4 L}=\frac{V_{0}}{L^{\prime}}$
$L ^{\prime}=\frac{4 L }{3} \frac{ V _{0}}{ V _{ C }}=\frac{4 L }{3} \sqrt{\frac{ B \cdot \rho_{1}}{\rho_{2} \cdot B }}( B$ is bulk modulus)
$=\frac{4 L }{3} \sqrt{\frac{\rho_{1}}{\rho_{2}}}$
$x =4$
