Ok. Note that $\displaystyle x(x-18)$ vanishes when x equals 0 or 18. Note also that $\displaystyle y=x(x-18)=x^2-18x$ is an equation of a parabola with a global minimum. That mean that $\displaystyle x(x-18)\geq 0$ when $\displaystyle x \in (-\infty,0]$ U $\displaystyle [18,+\infty,)$, that's your domain.
EDIT: I'm sorry in my first post, it should be "The domain is all the $\displaystyle x$ that make $\displaystyle x(x-18)\geq 0$."