Fill in according to the law, 3, 6, 9, 12, 15, how much
按规律填，3，6，9，12，15，多少
<gadget id="calculator">9 - 6</gadget>
<output>3</output>

<gadget id="calculator">15 + 3</gadget>
<output>18</output>

<result>18</result>