Perform four arithmetic operations on the four numbers 2, 3, 7, and 9, and use each number only once to make the result equal to 24.
2、3、7、9这四个数字进行四则运算，每个数只用一次，使其结果等于24．
<gadget id="calculator">3 * 7</gadget>
<output>21</output>

<gadget id="calculator">21 - 9</gadget>
<output>12</output>

<gadget id="calculator">2 * 12</gadget>
<output>24</output>

<result>24</result>