Answer
$45x^2-56x-45$
Work Step by Step
Using $(a+b)(c+d)=ac+ad+bc+bd$ or the product of $2$ binomials, the given expression, $
(5x-9)(9x+5)
$, is equivalent to
\begin{array}{l}\require{cancel}
5x(9x)+5x(5)-9(9x)-9(5)
\\\\=
45x^2+25x-81x-45
\\\\=
45x^2-56x-45
.\end{array}