#### Answer

$9r-s$

#### Work Step by Step

$\bf{\text{Solution Outline:}}$
To simplify the given radical expression, $
(3\sqrt{r}-\sqrt{s})(3\sqrt{r}+\sqrt{s})
,$ use the special product on multiplying the sum and difference of like terms.
$\bf{\text{Solution Details:}}$
Using the product of the sum and difference of like terms which is given by $(a+b)(a-b)=a^2-b^2,$ the expression above is equivalent
\begin{array}{l}\require{cancel}
(3\sqrt{r})^2-(\sqrt{s})^2
\\\\=
9r-s
.\end{array}