his question you must diagram of Silica, Metal Air, gap, Magnesite

Example $2\mathrm{.}21$ : A furnace wall made up of steel plate $10mm$ thick lined on the inside with silica brick $150mm$ thick on the outside with magnesite brick $150mm$ thick. The temperature on inside edge of the wall is $973K(700C)$ and on the outside is $288K(15C).$ Calculate the quantity of heat passed in watts per $m^2.$

It is required to reduce the heat flow to $1163\frac{W}{m^2}$ by means of an air gap between steel plate and magnesite brick. Estimate the width of this gap. Thermal conductivities in $\frac{W}{m*K}$ are $16\mathrm{.}86,1\mathrm{.}75,5\mathrm{.}23$ and $0\mathrm{.}033$ respectively for steel, silica brick, magnesite brick and air.