Chapter 5 - Section 5.3 - Special Products - Exercise Set - Page 367: 31

$9-r^2$

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