Write as a polynomial: (-3xy + a) (3xy + a)

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