Two cars, A and B, leave from A and B at the same time. After the first meeting, the two cars continue to move forward, and return immediately when they arrive at B and A respectively. When they meet for the second time, the two meeting places are 12 kilometers apart. Given that the speed ratio of the two vehicles is 3:4, find the distance between A and B.
甲乙两车分别从A、B两地同时相对开出，两车第一次相遇后继续前进，分别到达B、A两地立即返回，到第二次相遇，两次相遇地点相距12千米，已知两车速度比为3：4，求A、B两地路程．
<gadget id="calculator">3 + 4</gadget>
<output>7</output>

<gadget id="calculator">3 / 7</gadget>
<output>3/7 = around 0.428571</output>

<gadget id="calculator">15 / 7</gadget>
<output>15/7 = around 2.142857</output>

<gadget id="calculator">(15/7) - 2</gadget>
<output>1/7 = around 0.142857</output>

<gadget id="calculator">(3/7) - (1/7)</gadget>
<output>2/7 = around 0.285714</output>

<gadget id="calculator">12 / (2/7)</gadget>
<output>42</output>

<result>42</result>