rectangle with a width of 12 cm is formed by a rope with a length of 84 cm. What is the length of this rectangle?
用一根长84cm的绳子围成一个宽是12cm的长方形，这个长方形的长是多少．
<gadget id="calculator">84 / 2</gadget>
<output>42</output>

<gadget id="calculator">42 - 12</gadget>
<output>30</output>

<result>30</result>