Cars A and B start from places A and B at the same time, travel toward each other, and move back and forth between the two places. The first time they met at a place 3 kilometers away from A, car B first arrived at place A, then turned around immediately, and car B caught up with car A at a distance of 6 kilometers from place A. How many kilometers are there between A and B?
A、B两车分别从甲，乙两地同时出发，相向而行，在两地间往返运动．第一次在距甲地3千米处相遇，B车先到达甲地后，立即掉头，B车在距甲地6千米处追上A车．甲、乙两地相距多少千米？
<gadget id="calculator">6 - 3</gadget>
<output>3</output>

<gadget id="calculator">1 + 3</gadget>
<output>4</output>

<gadget id="calculator">3 * 4</gadget>
<output>12</output>

<result>12</result>