Angles b, c and a are the internal angles of a triangle. Then, they add up to 180°.
b + c + a = 180° (eq. 1)
Angles a and e are supplementary, then they add up to 180°.
e + a = 180° (eq. 2)
Subtracting equation 2 to equation 1, we get:
b + c + a = 180°
-
e + a = 180°
----------------------
b + c - e = 0°
or
b + c = e
