What is the concentration after traveling through the dermis?

Mass diffusion through multiple layers. Medicine can be applied through a patch and diffuse through the epidermis and dermis. Assume that the drug migration can be treated as diffusion through two unique solid layers with a constant concentration between layers. Let L1 be the thickness of the epidermis and L2 be the thickness of the dermis and the respective effective diffusivities within the layers can be denoted as DE and DD. No reaction occurs in the epidermis or dermis layers.
Derive an expression for the concentration profile.
Assume that the effective diffusivities for the epidermis and dermis are 0.9 x 10-10 and 200 x 10-10 m2/s respectively and respective layer thicknesses are 500 m and 1500 m. The surface removal rate is 200 pmol*m-2*s-1. The concentration at the surface near the patch is anticipated to be 10 nmol/mL.
What is the concentration after traveling through the dermis?