## Elementary Algebra

$81x^{2}-25y^{2}$
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 $(a+b)(a-b)$. $(9x+5y)(9x-5y)$ $=(9x)^{2}-(5y)^{2}$ $=81x^{2}-25y^{2}$