Looking at the triangles when they're in their alphabetical order, ΔABC and ΔRST, since they are congruent, that means that if you were to pick an angle in one triangle, it would be equal to the angle in the same place of the other triangle. So if we were to take the first letter from ΔABC it'd be A, and the first letter in ΔRST is R, so that means angles ∠A and ∠R should be congruent.
