$4.$ Imagine a rectangular wall which has dimensions of $30cm18cm$ and $20$ cm thickness. The temperatures at the inner and outer sides are $28$ and $5C,$ respectively. The wall is designed with a varying conductivity according to the temperature with this formulae: $k(T)=0,48+2,15{10}^{-2}T$ W$/$MK$.$ Assuming steady one dimensional heat transfer;

a$)$ Determine the rate of heat conduction through the wall

b$)$ Determine the rate if we change the temperatures at the inner and outer sides as $23$ and $10C,$ respectively.