I know that..but i need the domain in interval notation of that function
Well, if you know this is the case, then for what values of x is this inequality true? Just solve the inequality. Its fairly straightforward. Once you solve the inequality, the intervals become pretty obvious.
Ok. Note that vanishes when x equals 0 or 18. Note also that is an equation of a parabola with a global minimum. That mean that when U , that's your domain.
EDIT: I'm sorry in my first post, it should be "The domain is all the that make ."