Critical point of a cubic density equation of state

For $p=n{k}_{B}T-b{n}^2/2+c{n}^3/6$ with $b,c>0$, the first two density derivatives vanish at $n_c=b/c$, $k_B{T}_c=b^2/(2c)$ and $p_c=b^3/(6c^2)$. The scaled equation is $p/p_c=x^3-3x^2+3tx$, with $x=n/n_c$ and $t=T/T_c$.