Three concentric spherical metallic shells X, Y and Z of radius a, b and c respectively [a<b<c] have surface charge densities σ,-σ and σ,…

Three concentric spherical metallic shells $\displaystyle X, Y$ and $\displaystyle Z$ of radius $\displaystyle a, b$ and $\displaystyle c$ respectively $\displaystyle [a<b<c]$ have surface charge densities $\displaystyle \sigma,-\sigma$ and $\displaystyle \sigma$, respectively. The shells $\displaystyle X$ and $\displaystyle Z$ are at same potential. If the radii of $\displaystyle X \& Y$ are $\displaystyle 2$ cm and $\displaystyle 3$ cm , respectively. The radius of shell $\displaystyle Z$ is $\displaystyle \_\_\_\_$ cm.