Ratio PV/RT dieterici's equation

Hello

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

Determine the ratio PV/RT at the critical point for a fluid which obeys Dieterici’s equation

p(v-b)=RT exp(-a/RTv)



2. Relevant equations







3. The attempt at a solution

First ofall i don't really understand this notion of critical point.

I found an equation to find it that is

(dp/dv)=(d²p/dv²) at constant temperature T

I don't have any equation in my book so where did this get from ?

Using it I found that

V=2nb

P=a/(4b²e²)

T=a/4Rb

And then I don't know what to do



Thanks