## Elementary and Intermediate Algebra: Concepts & Applications (6th Edition)

$(a^2+9b^2)(a+3b)(a-3b)$
Using $x^2-y^2=(x+y)(x-y)$ or the factoring of the difference of 2 squares, then the factored form of the given expression, $a^4-81b^4 ,$ is \begin{array}{l} (a^2+9b^2)(a^2-9b^2) \\\\= (a^2+9b^2)(a+3b)(a-3b) \end{array}