Chapter 6 - Factoring, Solving Equations, and Problem Solving - 6.2 - Factoring the Difference of Two Squares - Problem Set 6.2 - Page 248: 44

$(9x^2+4y^2)(3x+2y)(3x-2y)$

Using $a^2-b^2=(a+b)(a-b)$, the factored form of the given expression, $ 81x^4-16y^4 ,$ is \begin{array}{l}\require{cancel} (9x^2+4y^2)(9x^2-4y^2) .\end{array} Since the second factor is still a difference of $2$ squares, using the same factoring technique, the completely factored form of the expression above is \begin{array}{l}\require{cancel} (9x^2+4y^2)(3x+2y)(3x-2y) .\end{array}