The diagram given shows that two angles in ∆ABC are congruent to two corresponding angles in ∆STU.
Invariably, the third unknown angle of both triangles would also be equal going by the third angle theorem.
Thus, based on the AA Similarity Theorem which says that two triangles are similar to each other if two corresponding angles of one is congruent to two angles in the other, ∆ABC ~ ∆STU.
