## Elementary Algebra

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