I recommend opening your favorite paint program, and using different colors to mark on the drawing as I have done below (see attached image). Note how angle DEC is in red and angle DEH is in blue. The two angles are adjacent, meaning they share the same ray (in this case, ray ED) and they form a straight angle.
Because they form a straight angle, or straight line, this means the two angles are supplementary. Supplementary angles always add to 180.
The slope is -1 because the number beside x is always the slope and the y-intercept is 1 because the number being added or subtracted from x is always the y intercept
In a regular polygon, each exterior angle equals each central angle. central angle = 360 / #sides #sides = 360 / central angle #sides = 360 / 60 #sides = 6