If QR ≅ QT that means ∠R= ∠T (angles opposite equal sides)
Given that PQRS is a parallelogram that means PQ║ ST(provided the parallelogram is extended to T) and SP║RQ
∠R≅∠Q( alternate ∠`s PQ║ST)
∠Q≅∠S(opposite angles of a parallelogram are equal)
If ∠Q≅∠R
∴∠Q≅∠T(Both are equal to ∠R)
∴∠S≅∠T( Both are equal to ∠Q)
Hope you understand!!