Chapter 5 - Section 5.3 - Special Products - Exercise Set - Page 367: 29

$9r^2-16$

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, $ (3r-4)(3r+4) ,$ is \begin{array}{l}\require{cancel} 3r(3r)-4(4) \\\\= 9r^2-16 .\end{array}