Chapter 5 - Section 5.3 - Special Products - Exercise Set - Page 367: 39

$r^6-4$

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, $ (r^3+2)(r^3-2) ,$ is \begin{array}{l}\require{cancel} r^3(r^3)-2(2) \\\\= r^6-4 .\end{array}