Two people, A and B, start from A to B at the same time. A rides a bicycle and B walks. At the same time, C starts from B to A. A meets C after riding 9 kilometers, and B meets C after riding 6 kilometers. If A's cycling speed is three times faster than B's walking speed, find the distance between A and B.
甲、乙两人同时从A地出发向B地前进，甲骑车，乙步行．与此同时，丙从B地出发向A地前进．甲骑9千米后与丙相遇，而乙走6千米后就与丙相遇．如果甲骑车的速度是乙步行速度的3倍，求A、B两地的距离．
<gadget id="calculator">6 + 6</gadget>
<output>12</output>

<result>12</result>