The length of a rectangle is 21 centimeters. If the length is reduced by 3 centimeters, the rectangle becomes a square. How many square centimeters does the area of the rectangle decrease?
一个长方形的长是21厘米，如果长减少3厘米，这个长方形就变成了正方形，那么长方形的面积减少了多少平方厘米？
<gadget id="calculator">21 - 3</gadget>
<output>18</output>

<gadget id="calculator">18 * 3</gadget>
<output>54</output>

<result>54</result>