Answer
$\sqrt[3]{25x^2}-81$
Work Step by Step
Using $(a+b)(a-b)=a^2-b^2$, or the product of the sum and difference of like terms, the expression $
(\sqrt[3]{5x}+9)(\sqrt[3]{5x}-9)
$ simplifies to
\begin{array}{l}
(\sqrt[3]{5x})^2-(9)^2
\\=
\sqrt[3]{25x^2}-81
.\end{array}
