Answer
$x^5+3x^3-x^2-3$
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^3-1)
$, is equivalent to
\begin{array}{l}\require{cancel}
x^2(x^3)+x^2(-1)+3(x^3)+3(-1)
\\\\=
x^5-x^2+3x^3-3
\\\\=
x^5+3x^3-x^2-3
.\end{array}
