The wall in front of the classroom is 8 meters long and 3 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，长8米，宽3米．墙上有一块黑板，面积是3平方米．现在要粉刷这面墙壁，要刷的面积是多少平方米？
<gadget id="calculator">8 * 3</gadget>
<output>24</output>

<gadget id="calculator">24 - 3</gadget>
<output>21</output>

<result>21</result>