1. **State the problem:** Factor the expression $x^2 - 9$. 2. **Recall the formula:** This is a difference of squares, which factors as $$a^2 - b^2 = (a - b)(a + b)$$ where $a = x$ and $b = 3$. 3. **Apply the formula:** $$x^2 - 9 = x^2 - 3^2 = (x - 3)(x + 3)$$ 4. **Explanation:** The difference of squares rule states that any expression that is one square minus another square can be factored into the product of the sum and difference of the square roots. 5. **Final answer:** $$\boxed{(x - 3)(x + 3)}$$