We divide each term by ${-6r^{2}}$ as follows. $$\begin{aligned} \displaystyle\frac{{18r^{20}+18r^{16}-54r^{14}}}{{-6r^{2}}} & = \frac{18r^{20}}{-6r^2}+\frac{18r^{16}}{-6r^2}+\frac{-54r^{14}}{-6r^2}\\ &={-{\frac{18}{6}}r^{18} - {\frac{18}{6}}r^{14}+{\frac{54}{6}}r^{12}}\\ &={-3r^{18}-3r^{14}+9r^{12}}\\ \end{aligned}$$