.
In a normal geometric language this statement is
In a parallelogram, the opposite angles are congruent.
See the lesson
- Opposite angles of a parallelogram
in this site.
Also, you have this free of charge online textbook on Geometry
GEOMETRY - YOUR ONLINE TEXTBOOK
in this site.
The referred lesson is the part of this textbook under the topic "Properties of parallelograms".
Save the link to this online textbook together with its description
Free of charge online textbook in GEOMETRY
https://www.algebra.com/algebra/homework/Triangles/GEOMETRY-your-online-textbook.lesson
to your archive and use it when it is needed.