Answer: ST = UX
Explanation:
The ASA indicates that the side (S) is between the two angles (A). The order is important. For the bottom triangle, the side between the red angles is segment ST. The upper triangle has the segment UX between the red angles. If we know that ST = UX, then we can use the ASA rule to prove the triangles are congruent.
Side note: ST = UX is the same as UX = ST. Furthermore, ST is the same as TS, and UX is the same as XU. So there are a few ways to fill in that blank.
