Answer
$-2a^4+3a^3-a-7$
Work Step by Step
Removing the grouping symbols and then combining like terms, the given expression, $
(5a^4+3a^3-a^2-2a-1)-(7a^4-a^2-a+6)
,$ simplifies to
\begin{array}{l}
5a^4+3a^3-a^2-2a-1-7a^4+a^2+a-6
\\\\=
(5a^4-7a^4)+3a^3+(-a^2+a^2)+(-2a+a)+(-1-6)
\\\\=
-2a^4+3a^3+0a^2-a-7
\\\\=
-2a^4+3a^3-a-7
.\end{array}
