Please help :)

A circular metal plate of 10 cm2 area is held parallel to and at a distance d above a solution, as in a surface potential measurement. The temperature is 25°C.
(a) First, assuming Ψ0 for the solution interface is 40 mV, calculate Ψ as a function of x, the distance normal to the surface on the air side. Assume that the air has 107 ions/cm3. (Ignore the plate for this part.)
(b) Calculate the contribution of the solution-air interface to the surface potential if the plate is now used as a probe in a surface potential measurement and is 1 mm from the surface.

(Addendum:

Question

Please help :)

A circular metal plate of 10 cm2 area is held parallel to and at a distance d above a solution, as in a surface potential measurement. The temperature is 25°C.
(a) First, assuming Ψ0  for the solution interface is 40 mV, calculate Ψ as a function of  x, the distance normal to the surface on the air side. Assume that the air has 107 ions/cm3. (Ignore the plate for this part.)
 (b) Calculate the contribution of the solution-air interface to the surface potential if the plate is now used as a probe in a surface potential measurement and is 1 mm from the surface.

(Addendum:

anonymous · Answer

pretty manga girl: hmmm, which book are you reading?

anonymous · Answer

it seems out of course for me 
;) lol

anonymous · Answer

|dw:1366651065042:dw|
we are looking for the potential at point P