AAS tells us that two traingles are coungruent if two angles are congruent and their corresponding opposite sides are congruent. From the picture we know that the segments and are parallel, hence and . Moreover, since the two angles and are opposite by a vertex, they are also congruent.
On the other hand, the picture tells us that . Therefore, since we already know that and that , we are only missing the information that to know that by AAS.