This is a link to a hard proof I couldn't do, try it out
Those 2 angles are equal, so the top and bottom are parallel, (alternate angles)
the top and bottom are also congruent. (given)
the two triangles are congruent, (2 sides and included angle test.)
so the two other sides are the same length
it would be easy enough to show they are parallel as well.
Hence it is a parallelogram.
that is the substance of the proof.