Answer:
Corresponding angles converse theorem
Explanation:
Angles x and y are corresponding angles.
This makes the lines parallel by the converse of that theorem.
This makes it similar by the corresponding angles converse theorem.
Definition of converse
the reverse of another
The corresponding angles theorem is for the congruency of the angles by parallel lines. Here we want to see if the lines are parallel given the congruency of the angles.
We can flip the operation, checking to see if the angles are congruent, which proves that the lines are parallel.
-Chetan k