In the above case, the formula of spherical capacitor consists of two concentric spherical conductors of radius, $r_1$ and $r_2$ is, $C=\frac{4\pi {\epsilon }_0{r}_1{r}_2}{r_1-r_2}$

Here $r_1=R$ and $r_2=2R$, now put the values in the above formula, 
$C=\frac{4\pi {\epsilon }_0\left(R\right)\left(2R\right)}{2R-R}=8\pi {\epsilon }_{0}R$