Chapter 4 - Quadratic Functions and Equations - 4-4 Factoring Quadratic Expressions - Lesson Check - Page 221: 3

$(x+9)(x-9)$

Using the factoring of the difference of $2$ squares which is given by $a^2-b^2=(a+b)(a-b),$ the given $\text{ expression, }$ $ x^2-81 ,$ is equivalent to \begin{align*} & (x)^2-(9)^2 \\&= (x+9)(x-9) \end{align*}