#### Answer

$9(x+3)(x-3)$

#### Work Step by Step

Factoring the $GCF=9$, then the given expression, $
9x^2-81
$, is
\begin{array}{l}
9(x^2-9)
.\end{array}Using $a^2-b^2=(a+b)(a-b)$ or the factoring of the difference of 2 squares, then the factored form of the expression, $
9(x^2-9)
$, is
\begin{array}{l}
9(x+3)(x-3)
.\end{array}