P(x)=x^3+ax^2+bx+c
P'(x)=3x^2+2ax+b

P(x)=(x/3+a/9)P'(x)+(2b/9)(3-a)x+c-ab/9

(2b/9)(3-a)x+c-ab/9=0
(2b/9)(3-a)x=ab/9-c
x=(ab-9c)/2b(3-a)