Recall that the domain of a function is all possibilities of x at which f(x) is defined:

where is this function defined? Well, you know that you can't take the sqrt of a negative number, s f(x) is defined every where that . (If x where any bigger than 3, 9-x^2 would be less than zero. A big no no unless we're considering complex numbers.)

In interval notation:

you can do the other now can't you?