Chapter 4 - Polynomials - 4.5 Multiplication of Polynomials - 4.5 Exercise Set - Page 269: 45

$x^2-9$

Using $(a+b)(c+d)=ac+ad+bc+bd$ or the product of $2$ binomials, the given expression, $ (x+3)(x-3) $, is equivalent to \begin{array}{l}\require{cancel} x(x)+x(-3)+3(x)+3(-3) \\\\= x^2-3x+3x-9 \\\\= x^2+0x-9 \\\\= x^2-9 .\end{array}