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