The school has a rectangular vegetable field with a length of 12 meters. If the length is increased by 4 meters, the area will increase by 28 square meters. What is the area of the original rectangular vegetable field in square meters?
学校有一块长方形菜地，长是12米，如果长增加了4米，面积就增加了28平方米，求原来的长方形菜地的面积是多少平方米？
<gadget id="calculator">28 / 4</gadget>
<output>7</output>

<gadget id="calculator">7 * 12</gadget>
<output>84</output>

<result>84</result>