Two people, A and B, ride bicycles from places A and B at the same time, and they meet for the first time at a distance of 7 kilometers from place B. After meeting, they continue to drive, and return immediately after reaching the destination. They met again at a distance of 4 kilometers from A, how many kilometers are there between A and B?
甲、乙两人骑自行车分别从A、B两地同时相向而行，第一次两人在距离B地7千米处相遇，相遇后，两人继续行驶，到达目的地后又立即返回，在距离A地4千米处又相遇了，求A、B两地相距多少千米？
<gadget id="calculator">7 * 3</gadget>
<output>21</output>

<gadget id="calculator">21 - 4</gadget>
<output>17</output>

<result>17</result>