If two squares with side length 3 cm are used to form a rectangle, what is the perimeter of the rectangle in cm?
用两个边长3厘米的正方形拼成一个长方形，这个长方形的周长是多少厘米．
<gadget id="calculator">3 + 3</gadget>
<output>6</output>

<gadget id="calculator">6 + 3</gadget>
<output>9</output>

<gadget id="calculator">9 * 2</gadget>
<output>18</output>

<result>18</result>