The domain is all the x-values you can use with this function. When dealing with a function like this, you need to ask yourself, what would make the bottom equal to 0? Whatever those numbers are, you do NOT want them in your domain.
A fundamental rule is that you cannot divide by 0 or have 0 as a denominator.
So, you solve x^2+6x=0 to find that x=0 or x=-6 would make the denominator 0.
The domain is everything BUT those two values, because they essentially break your function.