Construct a rhombus \(PQRS\) whose diagonals are \(PR = 6\text{ cm}\) and \(QS = 4\text{ cm}\). The first step taken is drawing the horizontal diagonal \(PR = 6\text{ cm}\) and finding its midpoint \(O\)
 
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What is the correct next geometric step to determine vertices \(Q\) and \(S\)?