From the statement of the problem, we know that:
• a train starts at City A and travels 2,158 km to City B,
,
• then it travels 3,793 km from City B to City C.
The distance between City A and City C is equal to the sum of the distance from City A to City B, and the distance from City B to City C. So the distance between City A and City C is 2,158 km + 3,793 km = 5951 km.
Looking at the answer of Clay:
2,158 + 3,793 = (2,158 + 7) + (3,793 + 7) = 2,165 + 3,800 = 5,965
We see that he added 7 km to each of the distances, that's the reason why he found a different a wrong result.