1. **State the problem:** We need to find the combined length of two line segments measured by two rulers. One ruler is horizontal measuring 15 cm, and the other is diagonal measuring 10 cm. 2. **Identify the lengths:** The horizontal ruler length is $15$ cm. 3. The diagonal ruler length is $10$ cm. 4. **Combine the lengths:** Since the problem asks for the combined length of the two line segments, we add their lengths directly: $$\text{Combined length} = 15 + 10 = 25$$ 5. **Final answer:** The combined length of the two line segments is $25$ cm.