If the length of a rectangle is 9 cm and the width is 3 cm, what is its perimeter in cm?
已知一个长方形的长是9厘米，宽是3厘米，它的周长是多少厘米？
<gadget id="calculator">9 + 3</gadget>
<output>12</output>

<gadget id="calculator">12 * 2</gadget>
<output>24</output>

<result>24</result>