We have, in this case, that:
Triangle RSU is congruent to triangle TUS because:
1. Side RS is congruent to side TU.
2. Both triangles share the same side (SU).
3. And because of the previous reasons, we can conclude that RU and TS are congruent too.
Therefore, these triangles are congruent because they have the same sides, then the rule is SSS (Side-Side-Side).
And we can conclude that triangle RSU is congruent to triangle TUS.