Answer
$81x^{2}-25y^{2}$
Work Step by Step
We can use the rule $(a+b)(a-b)=a^{2}-b^{2}$ in the simplification of the expression as the expression $(9x+5y)(9x-5y)$ is in the same format as $(a+b)(a-b)$.
$(9x+5y)(9x-5y)$
$=(9x)^{2}-(5y)^{2}$
$=81x^{2}-25y^{2}$
