To answer this question, we can see two parallel lines and one transversal. When we have this situation, we find many angles between the parallel lines and the other angles outside the parallel lines.
In this case, we have four angles between the parallel lines: We have interior angles here. We have an important theorem in Geometry that states: if we have two parallel lines, and a transversal that cut both lines, the alternate interior angles are congruent (they have the same measure).
The alternate interior angles are These alternate angles are congruent.
In summary, we need to remember that when we have two parallel lines, and a transversal line that cut them, we have that the alternate interior angles are congruent (they have the same measure). The angles