A wall with a thickness L is made of a material with a thermal conductivity that varies with its thickness x according to the equation, k = (ax + b) W/mK, where a and b are constants. If the heat flux applied at the surface of one end (x = 0) of the wall is 20 W/m², derive an expression for the temperature gradient and the temperature distribution across the wall thickness (between x = 0 and x = L). Use and define appropriate notations for the surface temperatures at each end of the wall.