Answer
$100x^4-9$
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, $
(10x^2+3)(10x^2-3)
$, is equivalent to
\begin{array}{l}\require{cancel}
(10x^2)^2-(3)^2
\\\\=
100x^4-9
.\end{array}