Use generators and relations to show that if $\displaystyle x\in\text{D}_{2n}$ which is not a power of r, then $\displaystyle rx=xr^{-1}$

$\displaystyle \text{D}_{2n}=<r,s:r^n=s^2=1, \ rs=sr^{-1}>$

Basically, x in this case is a flip. I am not sure what needs to proved. x is just a flip so by definition it satisfy $\displaystyle rx=xr^{-1}$