Answer
$2r^4+r^2-7$
Work Step by Step
Re-arranging the order of the terms, the given is equivalent to
$$
(2r^6-5r^4-10r^2+21)÷(r^2-3)
.$$
Using the long division method shown below, then
$$\begin{aligned}
&
(2r^6-5r^4-10r^2+21)÷(r^2-3)
\\&=
2r^4+r^2-7+\frac{\color{red}{0}}{\color{blue}{r^2-3}}
\\&=
2r^4+r^2-7
.\end{aligned}
$$
