Answer
$x^2-36$
Work Step by Step
Using $(a+b)(c+d)=ac+ad+bc+bd$ or the product of $2$ binomials, the given expression, $
(x+6)(x-6)
$, is equivalent to
\begin{array}{l}\require{cancel}
x(x)+x(-6)+6(x)+6(-6)
\\\\=
x^2-6x+6x-36
\\\\=
x^2+0x-36
\\\\=
x^2-36
.\end{array}