Answer
$3a^4-13a^3-13a^2-4$
Work Step by Step
By combining like terms, the given expression, $
(3a^4-9a^3-7)-(4a^3+13a^2-3)
$, simplifies to
\begin{array}{l}\require{cancel}
3a^4-9a^3-7-4a^3-13a^2+3
\\\\=
3a^4+(-9a^3-4a^3)-13a^2+(-7+3)
\\\\=
3a^4-13a^3-13a^2-4
.\end{array}
