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