The perimeter of a rectangle is 18 cm, its length is 6 cm, and its width is in cm.
长方形周长18厘米，它的长是6厘米，宽是多少厘米．
<gadget id="calculator">18 / 2</gadget>
<output>9</output>

<gadget id="calculator">9 - 6</gadget>
<output>3</output>

<result>3</result>