1. **Problem statement:** Given a figure where $QR = 15$ and $QS = 45$, find the length of $RS$. 2. **Understanding the problem:** Points $Q$, $R$, and $S$ are on a line or form a triangle. Since $QS$ is longer than $QR$, and $RS$ is the segment between $R$ and $S$, we can use the segment addition postulate. 3. **Formula used:** The segment addition postulate states: $$QS = QR + RS$$ 4. **Substitute known values:** $$45 = 15 + RS$$ 5. **Solve for $RS$:** $$RS = 45 - 15$$ $$RS = 30$$ 6. **Answer:** The length of $RS$ is $30$ units. This means that the segment $RS$ is 30 units long, completing the total length $QS$ of 45 units.