This is an example of the difference of squares.
[tex]a^2 - b^2 = (a-b)(a+b)[/tex]
So to find the product of (9y² – 4x)(9y² + 4x), we use the formula for the difference of squares.
[tex](9y^2 - 4x)(9y^2 + 4x)\\\\=(9y^2)^2 -(4x)^2\\\\=\boxed{\bf{81y^4 - 16x^2}}[/tex]
