A starts from A and B travels toward each other at a uniform speed at the same time. The first time they meet A is 6 kilometers away. They continue to move forward and return immediately after reaching the starting point of the other party. They meet for the second time at 3 kilometers away from B. , what is the distance between A and B in kilometers?
甲从A地，乙从B地同时以均匀的速度相向而行，第一次相遇A地6千米，继续前进，到达对方起点后立即返回，在离B地3千米处第二次相遇，则A、B两地相距多少千米．
<gadget id="calculator">6 * 3</gadget>
<output>18</output>

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

<result>15</result>