A rectangular plank is twice as long as it is wide and has a perimeter of 84 cm. What is the area of this rectangular board in square centimeters?
一块长方形木板，长是宽的2倍，周长是84厘米．这个长方形木板的面积是多少平方厘米．
<gadget id="calculator">84 / 2</gadget>
<output>42</output>

<gadget id="calculator">2 + 1</gadget>
<output>3</output>

<gadget id="calculator">42 / 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>