Q$3$: A constant$-$area steel rod between two surfaces at temperatures $T_1$ and $T_2$ is shown in Figure $3\mathrm{.}7.$ If the external temperature, $T_{}(x),$ is a function of the coordinate $x,$ find the general steady$-$state solution of the fin temperature $T(x)$ for $($a$)T_l=T_2$ and $($b$)T_l{T}_2.$