Answer:
AA Similarity Postulate
Step-by-step explanation:
we know that
If two figures are similar, then the ratio of its corresponding sides is proportional and its corresponding angles are congruent
step 1
Verify the proportion of the corresponding sides

substitute

----> is true
Corresponding sides are proportional
Triangle PQR is similar to Triangle PST
That means
Corresponding angles must be congruent
side QR is parallel side ST
and
----> by corresponding angles
--> by corresponding angles
so
PQR is similar to PST by AA Similarity Postulate