For a rectangle, if the length increases by 3cm, the area increases by 12cm**2, and if the length decreases by 3cm, a square is obtained. What is the area of this rectangle in cm**2.
一个长方形，如果长增加3cm，面积就增加12cm**2，如果长减少3cm，就得到一个正方形．这个长方形面积是多少cm**2．
<gadget id="calculator">12 / 3</gadget>
<output>4</output>

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

<gadget id="calculator">7 * 4</gadget>
<output>28</output>

<result>28</result>