1. Problem: Two right triangles $\triangle PQR$ and $\triangle ABC$ are given. 2. In $\triangle PQR$ the right angle is at $Q$, so $\angle Q=90^\circ$. 3. Sides $PQ$ and $QR$ are marked equal to $AB$ and $BC$ respectively, i.e. $PQ=AB$ and $QR=BC$. 4. Formula and rules: For right triangles the Leg-Leg (LL) postulate states that if the two legs of one right triangle are congruent to the two legs of another right triangle then the triangles are congruent. 5. Other congruence criteria include $SSS$, $SAS$, $ASA$, $AAS$, and for right triangles $HL$ (Hypotenuse-Leg), but LL is specific to two equal legs. 6. Identification: The legs are the sides adjacent to the right angle. 7. In $\triangle PQR$ the legs are $PQ$ and $QR$. 8. In $\triangle ABC$ the legs are $AB$ and $BC$. 9. Since $PQ=AB$ and $QR=BC$ both pairs of corresponding legs are equal. 10. Conclusion: By the Leg-Leg (LL) congruence postulate for right triangles we have $\triangle PQR \cong \triangle ABC$. 11. Final answer: Leg-Leg (LL) congruence postulate.