Answer
$x^2-9$
Work Step by Step
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}