Use the four numbers 2, 3, 3, and 9 to calculate 24 points, and express it with a comprehensive formula.
用2、3、3、9这四个数算出24点，并用综合算式表示出来．
<gadget id="calculator">2 * 9</gadget>
<output>18</output>

<gadget id="calculator">18 + 3 + 3</gadget>
<output>24</output>

<result>24</result>