There are 12 judgment questions in an intelligence test, 3 points are awarded for each correct answer, and 2 points are deducted for each incorrect answer. Xiaoqiang finished answering 12 questions and only scored 21 points. How many questions did Xiaoqiang answer correctly?
一次智力测验有12道判断题，每答对一道得3分，每答错一道扣2分．小强答完了12道题，只得了21分．小强答对了多少道题．
<gadget id="calculator">12 * 3</gadget>
<output>36</output>

<gadget id="calculator">36 - 21</gadget>
<output>15</output>

<gadget id="calculator">3 + 2</gadget>
<output>5</output>

<gadget id="calculator">15 / 5</gadget>
<output>3</output>

<gadget id="calculator">12 - 3</gadget>
<output>9</output>

<result>9</result>