corresponding angles post
if two parallel lines are cut by a transversal, then the pairs of corresponding angles are congruent.
converse of corresponding angles post
if two coplanar lines are cut by a transversal so that a pair of corresponding angles are congruent, then the two lines are parralel. (if the coresponding angles are congruent, that means that the two lines cut by the transversal are parralel.)
---(this is how the reasonings are probably going to be like in a proof)----
-vert angles congruence/alt ext angles congruene
-conv of corres angle theorym
through a point p not on a line l, there is exactly one line parallel to l.