smc

Denote the $2$ roots of this quadratic as $r_1$ and $r_2$. Note that $(r_1-r_2{)}^2=(r_1+r_2{)}^2-4r_1{r}_2$. By Vieta's Formula's, $r_1+r_2=7$, and $r_1{r}_2=-9$. Thus, $r_1-r_2=\sqrt{49+4\cdot 9}=\boxed{\sqrt{85}}$.