Chapter 5 - Section 5.3 - Special Products - Exercise Set - Page 367: 25

$x^2-9$

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