I have this problem that seems relatively easy but i seem to be messing up somewhere.

First i find x-ints/y-ints

x^3-9x

x(x+3)(x-3)

x=0 x=3 x=-3

y=0

Now the troubling part

Critical Point: F'(x)=0

3x^2-9=0

I use simple quadratic formula and get x values of 18 and -18 (these values don't seem correct) i can't seem to find out what i am doing wrong, any help on this is greatly appreciated.