Question

reference angle and coterminal angle of 5pi 5pi/3 , - 6pi/5

Answer 1

Reference angle of 5pi is pi. Just draw a diagram - 5pi is 4pi + pi - the 4pi is just two complete revolutions, so the remaining angle is just one pi. (Reference angles are the angle between a given line and the horizontal). Coterminal angles of any angle are just the angle with some multiple of 2pi added to it.
(5/3) pi falls into the bottom left quadrant. The coterminal angle would be -(2/3)pi.
-6/5*pi would have a coterminal angle of (1/5)pi

Your sign convention may be the reverse of mine.