Answer
$(x+9+y)(x+9-y)$
Work Step by Step
Place the terms into two groups. Then use trial and error to factor the terms inside the parentheses. Next use the formula $a^2-b^2=(a+b)(a-b)$ to factor completely.
$x^2+18x+81-y^2$
$(x^2+18x+81)-y^2$
$(x+9)^2-y^2$
$(x+9+y)(x+9-y)$
