If the length of a rectangle is 9 decimeters and its width is 3 decimeters, what is its perimeter?
一个长方形的长是9分米，比宽长3分米，它的周长是多少分米．
<gadget id="calculator">9 - 3</gadget>
<output>6</output>

<gadget id="calculator">9 + 6</gadget>
<output>15</output>

<gadget id="calculator">15 * 2</gadget>
<output>30</output>

<result>30</result>