Answer
$x^4-4x^2-21$
Work Step by Step
Using $(a+b)(c+d)=ac+ad+bc+bd$ or the product of $2$ binomials, the given expression, $
(x^2+3)(x^2-7)
$, is equivalent to
\begin{array}{l}\require{cancel}
x^2(x^2)+x^2(-7)+3(x^2)+3(-7)
\\\\=
x^4-7x^2+3x^2-21
\\\\=
x^4-4x^2-21
.\end{array}
