For the power system given in Problem $7\mathrm{.}12,$ a three$-$phase short circuit occurs at bus $2,$ where the prefault voltage is $525$ kV$.$ Prefault load current is neglected. Determine the $($a$)$ Th$$venin equivalent at the fault, $($b$)$ sub$-$transient fault current in per unit and in kA rms$,$ and $($c$)$ contributions to the fault from lines $1-2,2-3,$ and $2-4..$