You can see by the markings that ST is congruent to AY, and that TA is congruent to YS. Those are 2 sides, so so far we have SS. One important thing that comes up WAY TOO OFTEN IN GEOMETRY TO IGNORE is the fact that both of those triangles share the side SA. This is called the reflexive property. The theorem for congruency between these 2 triangles is SSS.
