if you use the formula $$\large R=\rho \frac{L}{A}=\rho\frac{L}{\pi r^2}$$ its given that $$\rho$$ remains same $$L'=2L,r'=2r$$ so $$\large new \: R=R'=\rho \frac{2L}{\pi (2r)^2}=....?$$ so, can u find R' in terms of R ?