The Depletion Layer in Thermal Equilibrium

[physics0]

1. The problem statement, all variables and given/known data

question: b,c,d,e

2. Relevant equations

possion's equation

3. The attempt at a solution

I try to solve the problem 1 of Chap.29 on the text:
Solid State Physics,Ascroft & Mermin

but when I cannot solve after quesion (a).

so, anyone, pls help me or give some hints about question b,c,d,e.

thank you

[physics0]

pls see the attachments about the problem