Appendix A - Review - A.3 Polynomials - A.3 Assess Your Understanding - Page A30: 78

$(x+3)(x-3)$

Since $9=3^2$, then $x^2-9=x^2-3^2$ Use special formula $a^2-b^2=(a+b)(a-b)$. We have $a=x$ and $b=3$. Thus, $x^2-9=(x+3)(x-3)$ Hence, the completely factored form of the given expressions is $(x+3)(x-3)$.