The perimeter of a rectangular vegetable field is 18 meters, the width is 2 meters, and the length is in meters.
一块长方形菜地的周长是18米，宽是2米，长是多少米．
<gadget id="calculator">18 / 2</gadget>
<output>9</output>

<gadget id="calculator">9 - 2</gadget>
<output>7</output>

<result>7</result>