The wall behind the classroom is 7 meters long and 3 meters wide. There is a blackboard on the wall. The area is 3 square meters. Now this wall is to be painted, what is the area to be painted?
教室后面的墙壁，长7米，宽3米，墙上有块黑板，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？
<gadget id="calculator">7 * 3</gadget>
<output>21</output>

<gadget id="calculator">21 - 3</gadget>
<output>18</output>

<result>18</result>