Answer:
From the graph attached, we know that
by the corresponding angle theorem, this theorem is about all angles that derive form the intersection of one transversal line with a pair of parallels. Specifically, corresponding angles are those which are placed at the same side of the transversal, one interior to parallels, one exterior to parallels, like
and
.
We also know that, by definition of linear pair postulate,
and
are linear pair. Linear pair postulate is a math concept that defines two angles that are adjacent and for a straight angle, which is equal to 180°.
They are supplementary by the definition of supplementary angles. This definition states that angles which sum 180° are supplementary, and we found that
and
together are 180°, because they are on a straight angle. That is, 
If we substitute
for
, we have
, which means that
and
are also supplementary by definition.