Coterminal, first and foremost, means that it ends at the same point that another angle ends. So, we can say that angle M is equivalent to -620°. Now, we can all agree that a circle has 360° in it, so moving an angle 360° in either direction doesn't change where the angle ends, because it just moved one full trip around the circle, and is right back where it started. Overall, we need to put -620° in between 0° and 360° by adding 360° until it's between those two angles:
-620°+360° = -260°
-260°+360° = 100°
Now, 100° is between 0° and 360°, so we can say that angle M must be equal to 100°.
