"(9y-a)(9y+a)"

"(9y-a)(9y+a)" can be simplified using the formula for the difference of squares, which states that (a+b)(a-b) is equal to a^2 - b^2. In this case, a = 9y and b = a. Therefore, the expression is equal to (9y)^2 - a^2, which simplifies to 81y^2 - a^2.