1. **Problem:** Find the length of diagonal QS in rectangle PQRS given that diagonal PR = 12. 2. **Recall:** In a rectangle, the diagonals are congruent. This means $$PR = QS$$. 3. **Apply the property:** Since $$PR = 12$$, then $$QS = 12$$. 4. **Explanation:** A rectangle has equal diagonals because it is a parallelogram with right angles, so the diagonals are congruent. **Final answer:** $$QS = 12$$.