I would convert to degrees first seems to make it easier. [tex] \frac{-13 \pi }{5} [/tex]×[tex] \frac{180}{ \pi } [/tex] = -468 by adding 360 twice we get the coterminal angle 252
252 is in the 3rd quadrant so 180 is the closest angle on the x-axis ref angle = 252 - 180 = 72
If necessary convert back to radians... [tex]72( \frac{ \pi }{180} )= \frac{2 \pi }{5} [/tex]
