How do you find the reference angle for -(11pi)/3?

$\frac{\pi}{3}$
Reference angle of $\frac{- 11 \pi}{3}$ is the same as reference angle of $\left(- 5 \frac{\pi}{3} - 2 \pi\right) .$
Reference angle (or arc) of $\frac{- 5 \pi}{3}$ is $\left(\frac{\pi}{3}\right)$ (co-terminal)