If the width of a rectangle is 3 cm and the length is twice the width, what is the area of the rectangle in square centimeters?
一个长方形的宽是3厘米，长是宽的2倍，长方形的面积是多少平方厘米．
<gadget id="calculator">3 * 2</gadget>
<output>6</output>

<gadget id="calculator">6 * 3</gadget>
<output>18</output>

<result>18</result>