The wall in front of the classroom is 8 meters long and 3 meters high. There is a blackboard with an area of 9 square meters on the wall. If you want to paint this wall, how many square meters should you paint?
教室前面的墙壁长是8米，高是3米，墙上有一块面积是9平方米的黑板，如果要粉刷这面墙壁，要粉刷的面积是多少平方米？
<gadget id="calculator">8 * 3</gadget>
<output>24</output>

<gadget id="calculator">24 - 9</gadget>
<output>15</output>

<result>15</result>