A rectangular vegetable field has a circumference of 24 meters and a width of 4.5 meters. What is the length in meters?
一块长方形的菜地，周长是24米，宽是4.5米，长是多少米．
<gadget id="calculator">24 / 2</gadget>
<output>12</output>

<gadget id="calculator">12 - 4.5</gadget>
<output>7.5</output>

<result>7.5</result>