Question

Reference angle for -19pi/6? 

Answer 1

$\bf \cfrac{19}{6}\implies 3+\cfrac{1}{6} \\ \quad \\\\ \cfrac{-19\pi }{6}\implies -\left(3\pi +\cfrac{1\pi }{6} \right)\impliedby clockwise \\ \quad \\\\ or\implies -\left(2\pi +\pi +\cfrac{\pi }{6} \right)$

go that much "clockwise", you'll end up at the 2nd quadrant,
the reference angle, is its reflective angle on the 1st quadrant