The difference of the roots of $x^2-7x-9=0$ is:
$\textbf{(A) \ }+7  \qquad \textbf{(B) \ }+\frac{7}{2} \qquad \textbf{(C) \ }+9 \qquad \textbf{(D) \ }2\sqrt{85} \qquad \textbf{(E) \ }\sqrt{85}$
Denote the $2$ roots of this quadratic as $r_1$ and $r_2$. Note that $(r_1-r_2)^2=(r_1+r_2)^2-4r_1r_2$. By [Vieta's Formula's](https://artofproblemsolving.com/wiki/index.php/Vieta%27s), $r_1+r_2=7$, and $r_1r_2=-9$. Thus, $r_1-r_2=\sqrt{49+4\cdot 9}=\boxed{\textbf{(E)}\ \sqrt{85}}$.