Consider the expression [tex] (9y^2- 4x)(9y^2 + 4x) .[/tex] You can see that in both brackets are the same terms [tex] 9y^2 [/tex] and [tex] 4x [/tex], but signs are different. So you can use formula for the difference of squares:
[tex] a^2-b^2=(a-b)(a+b). [/tex]
According to this formula,
[tex] (9y^2-4x)(9y^2+4x)=(9y^2)^2-(4x)^2=9^2\cdot (y^2)^2-4^2\cdot x^2=81y^4-16x^2. [/tex]
Answer: correct choice is B.
