Two people, A and B, set off from A and B at the same time by bicycle, and go back and forth between A and B. They meet for the first time at a distance of 6 kilometers from B. They return immediately after arriving at the end point. The distance between the second encounter is At a distance of 3 kilometers from A, find the distance between A and B.
甲乙二人骑自行车分别从A、B两地同时出发，往返于A、B两地之间，第一次相遇于距B地6千米处，各自到达终点后立即返回，第二次相遇距A地3千米处，求A、B两地的距离．
<gadget id="calculator">6 * 3</gadget>
<output>18</output>

<gadget id="calculator">18 - 3</gadget>
<output>15</output>

<result>15</result>