$2\mathrm{.}18$ Polyethylene with Er$=2\mathrm{.}25$ is used as the insulation material in a lossless coaxial line with a characteristic impedance of $50$ohm. The radius of the inner conductor is $1\mathrm{.}2$mm$.($a$)$ What is the radius of the outer conductor? $($b$)$ What is the phase velocity of the line?