$R _{ net }=\frac{ R _{1} R _{2}}{ R _{1}+ R _{2}}$

$\frac{\rho \ell}{2 A }=\frac{\rho_{1} \frac{\ell}{ A } \times \rho_{2} \frac{\ell}{ A }}{\rho_{1} \frac{\ell}{ A }+\rho_{2} \frac{\ell}{ A }}$

$\frac{\rho}{2}=\frac{6 \times 3}{6+3}=2$

$\rho=4$