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