Question 39854
Domain means "all allowable values of x."
For y = sqrt(x^2 - 9), 
notice that what's under the square root sign can never be negative, so we have to choose our x's such that x^2 - 9 is greater than or equal to zero...so that's what we write...
x^2 - 9 ≥ 0
x^2 ≥ 9
|x| ≥ 3
or another way to write this is
x ≥ 3 or x ≤ -3
Graphically, there would be no graph between -3 and 3, since x doesn't exist there, so to speak...those numbers are not in the domain of the function...