From the figure. a and b are equal because the lines are parallel. The jargon is alternate interior angles are equal.
The two remote interior angles are equal to the exterior angle. That interprets to
b + 2a = 84o But b = a
a + 2a = 84o
3a = 84
a = 84/3
a = 28 <<<< answer
b = 28 <<<< answer
2a = 56 <<<< answer.
