Paint a classroom with a length of 9m, a width of 6m, and a height of 3m. The area of doors and windows is 20.5㎡. What is the formula for representing the area of the classroom?
粉刷一间长9m，宽6m，高3m的教室，门窗面积是20.5㎡，表示教室粉刷面积的式子是多少
<gadget id="calculator">9 * 6</gadget>
<output>54</output>

<gadget id="calculator">6 * 3 * 2</gadget>
<output>36</output>

<gadget id="calculator">9 * 3 * 2</gadget>
<output>54</output>

<gadget id="calculator">54 + 36 + 54</gadget>
<output>144</output>

<gadget id="calculator">144 - 20.5</gadget>
<output>123.5</output>

<result>123.5</result>