A square with a side length of 9 cm can be surrounded by a piece of iron wire. If it is surrounded by a rectangle with a length of 12 cm, what is the width of the rectangle in cm?
用一根铁丝可以围成一个边长为9厘米的正方形，如果把它围成一个长是12厘米的长方形，长方形的宽是多少厘米？
<gadget id="calculator">9 * 4</gadget>
<output>36</output>

<gadget id="calculator">36 / 2</gadget>
<output>18</output>

<gadget id="calculator">18 - 12</gadget>
<output>6</output>

<result>6</result>