Analogous to the heat conduction through multiple layers, derive the molecular mass diffusion of a drug, A through two layers $($skin and muscle$)$ of thicknesses $x_1$ and $x_2,$ respectively, and diffusivity of the drug being $D_{A1}$ and $D_{A2}$ respectively. If the concentration of the patch drug on this slab geometry was $C_{A0}$ at the surface of the skin, and it is $C_{A1}$ at the end of the muscle layer, derive the concentration profile $($particular solution$)$ of the drug across both the layers, under steady$-$state conditions. $(40$ points$)$

FIND THE PARTICULAR AND GENERAL SOLUTION!!!