A wall behind the classroom is 7 meters long and 3 meters wide. There is a blackboard on the wall with an area of 4 square meters. Now to paint this wall, how many square meters is the area to be painted?
教室后面的一面墙壁，长7米，宽3米．墙上有一块黑板，面积是4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 * 3</gadget>
<output>21</output>

<gadget id="calculator">21 - 4</gadget>
<output>17</output>

<result>17</result>