1. The problem is to factor the expression $x^2 - 9$. 2. Recognize that this is a difference of squares, which follows the formula $a^2 - b^2 = (a - b)(a + b)$. 3. Here, $x^2$ is $a^2$ and $9$ is $3^2$, so $a = x$ and $b = 3$. 4. Applying the formula, we get: $$x^2 - 9 = (x - 3)(x + 3)$$ 5. This is the fully factored form of the expression.