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

$a^2-9b^2$
Using $(a+b)(a-b)=a^2-b^2$ or the product of the sum and difference of like terms, the given expression, $(a-3b)(a+3b) ,$ simplifies to \begin{array}{l}\require{cancel} (a)^2-(3b)^2 \\\\= a^2-9b^2 .\end{array}