Final answer:
The domain of the function x^2 - 9 is all real numbers, represented as (-∞, ∞) in interval notation.
Step-by-step explanation:
To find the domain of x^2 - 9, we need to determine the set of all possible values of x that can be plugged into the function without causing any mathematical inconsistencies, such as division by zero or taking the square root of a negative number. Since x^2 is the square of x, and subtracting 9 does not impose any additional restrictions, x can be any real number. Therefore, the domain of the function x^2 - 9 is all real numbers, which can be represented as (-∞, ∞) in interval notation.