There are 21 questions in one test paper, 8 points are awarded for each correct answer, and 6 points are deducted for each incorrect answer. Xiao Ming finished answering all the questions, but got zero points. How many questions did he answer correctly?
一张试卷共有21道题，答对一道得8分，答错一道扣6分．小明答完了所有的题目，却得了零分，他答对了多少道题．
<gadget id="calculator">21 * 8</gadget>
<output>168</output>

<gadget id="calculator">8 + 6</gadget>
<output>14</output>

<gadget id="calculator">168 / 14</gadget>
<output>12</output>

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

<result>9</result>