Answer:
a^2 - 9x^2y^2
Step-by-step explanation:
(−3xy + a)(3xy + a) =
= (a - 3xy)(a + 3xy)
This is the product of a sum and a difference: (a + b)(a - b) = a^2 - b^2
= a^2 - 9x^2y^2
