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