When you have two parallel lines being cut by a transversal, you can use the next statements for the angles:
The measurement of the angles in green is the same (are congruents)
The measurement of the angles in blue is the same (are congruents)
An angle in green is a complementary angle with an angle in blue. (The addition of the two angles has to be 180º)
You know that:
As the angle 3 is congruent with angle 2:
