What are the critical values of y'=x^3-x^2 ?

Assuming you mean the y-value associated with the critical point(s)... 
y''=3x^2-2x

y''=0 implies
x(3x-2)=0
leads to x=0 or x=2/3
use these values of x in y' to get critical values:
y'(2)=8-4=4
y'(2/3)=8/27-4/9=-4/27