The wall in front of the classroom is 7 meters long and 4 meters wide. There is a blackboard on the wall with an area of 3 square meters. Now to paint this wall, how many square meters is the area to be painted?
教室前面的墙壁，长7米，宽4米．墙上有一块黑板，面积是3平方米．现在要粉刷这面墙壁，要粉刷的面积是多少平方米？
<gadget id="calculator">7 * 4</gadget>
<output>28</output>

<gadget id="calculator">28 - 3</gadget>
<output>25</output>

<result>25</result>