If a rectangle is extended by 4 meters, it becomes a square with an area of 49 square meters. What is the area of the original rectangle in square meters?
一个长方形，将宽延长4米，就变成了一个正方形，面积是49平方米，原来长方形的面积是多少平方米．
<gadget id="calculator">7 - 4</gadget>
<output>3</output>

<gadget id="calculator">3 * 7</gadget>
<output>21</output>

<result>21</result>