jmc

Dividing both sides of the equation $9x^2-18x-720=0$ by $9$, we have $$x^2-2x-80=0.$$The square which agrees with $x^2-2x-80$ except for the constant term is $(x-1{)}^2$, which is equal to $x^2-2x+1$ and thus to $(x^2-2x-80)+81$.

Therefore, by adding $81$ to each side, Marina rewrote the equation $x^2-2x-80=0$ as $$(x-1{)}^2=81.$$We have $r=-1$ and $s=\boxed{81}$.