Answer
$7a^3-a^2+13a+7$
Work Step by Step
By combining like terms, the given expression, $
(3+6a+7a^2+a^3)+(4+7a-8a^2+6a^3)
$, simplifies to
\begin{array}{l}
(a^3+6a^3)+(7a^2-8a^2)+(6a+7a)+(3+4)
\\\\=
7a^3-a^2+13a+7
.\end{array}
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