Step-by-step explanation: There are multiple ways with dealing with this problems. But since Angle A is on the exterior let limit it to just 3.
Theorem #1: A transversal crossing two parallel lines make alternate exterior angles congruent.
You can also use the vertical angles theorem, Angles that share same vertex has same degree measure therefore, congruent
And theorem #3: A transvesal crossing two parallel lines make corresponding or same side angles congruent.
So you can put the black pont two places down from angle a on the left side, the last and lowest angle on the right side, or diagonally across angle a which is on the right side.
