a and a' are vertical angles, therefore a' = a
We know: The sum of the measures of triangle is always 180°.
Therefore: a' + b + d = 180° → d = 180° - a - b.
d and c are adjacent angles, therefore d + c = 180°
substitute
180° - a - b+ c = 180° |-180°
-a - b + c = 0 |+a |+b
c = a + b