#### Answer

$x-81$

#### Work Step by Step

Using $(a+b)(a-b)=a^2-b^2$, or the special product of the sum and difference of like terms, the given expression, $
(\sqrt{x}+9)(\sqrt{x}-9)
,$ simplifies to
\begin{array}{l}\require{cancel}
(\sqrt{x})^2-(9)^2
\\\\=
x-81
.\end{array}
Note that all variables represent positive real numbers.