$($a$)$ For the system given below, two gray surfaces are surrounded by refractory walls.

Derive the equation for the net heat loss from the surface $1$ to surface $2,$ q$12.$

$($b$)$ From your result for part $($a$),$ derive the following equation $(14\mathrm{.}33)$ from the textbook.

$\frac{?}{\text{b}\text{a}\text{r}}(F{)}_{12}=\frac{A_2-A_1{F}_{12}^2}{A_1+A_2-2A_1{F}_{12}}$