How to find partial derivative of dependent variable at one point

I am using diffusion time dependent model with two dependent variables cp and cb.
My equation of cb requires me to input "partial derivative of cp at x=0"

i,e cb=k*pd(cp,x)|(at x=0)

k is constant.

And in calculation of cp my boundary conditions for it are at x=0 and L cp=cb. There is no cb in the differential equation of cp except for boundary conditions.

How am I supposed to calculate partial derivative of cp at x=0?
Please Help
Thank You