If the width of a rectangle is increased by 4 cm, the area will increase by 28 cm2 to become a square. What is the area of the original rectangle?
一个长方形，它的宽增加4厘米，面积就增加28平方厘米，正好变成一个正方形，原来的长方形的面积是多少？
<gadget id="calculator">28 / 4</gadget>
<output>7</output>

<gadget id="calculator">7 - 4</gadget>
<output>3</output>

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

<result>21</result>