The distance between A and B is 21 kilometers. A starts from A and travels 4 kilometers per hour. At the same time, B starts from B and travels towards each other and travels 3 kilometers per hour. After meeting on the way, the two turned their backs. They return immediately after arriving at their destination, and meet for the second time on the way. How many miles is the distance between the two meeting points?
A、B两地相距21千米，甲从A地出发，每小时行4千米，同时乙从B地出发相向而行，每小时行3千米．在途中相遇以后，两人又背而行．各自到达目的地后立即返回，在途中二次相遇．两次相遇点间相距多少千米？
<gadget id="calculator">3 + 4</gadget>
<output>7</output>

<gadget id="calculator">21 / 7</gadget>
<output>3</output>

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

<gadget id="calculator">21 - 12</gadget>
<output>9</output>

<gadget id="calculator">4 * 9</gadget>
<output>36</output>

<gadget id="calculator">36 - 21 - 9</gadget>
<output>6</output>

<result>6</result>