## Elementary Algebra

$x-81$
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.