Answer
$x^{20}-9$
Work Step by Step
Using $(a+b)(a-b)=a^2-b^2$ or the product of the sum and difference of like terms, the given expression, $
(x^{10}+3)(x^{10}-3)
$, is equivalent to
\begin{array}{l}\require{cancel}
(x^{10})^2-(3)^2
\\\\=
x^{20}-9
.\end{array}