It is known that the perimeter of the rectangle is 84 decimeters, and the length is twice the width. What is the area of the rectangle in square decimeters?
已知长方形的周长是84分米，长是宽的2倍，长方形的面积是多少平方分米？
<gadget id="calculator">2 + 1</gadget>
<output>3</output>

<gadget id="calculator">84 / 2 / 3</gadget>
<output>14</output>

<gadget id="calculator">14 * 2</gadget>
<output>28</output>

<gadget id="calculator">28 * 14</gadget>
<output>392</output>

<result>392</result>