Xiao Ming has 6 stamps. After Xiao Gang gives him 3 stamps, the number of stamps he owns is twice that of Xiao Ming's stamps. How many stamps did Xiaogang have?
小明有6张邮票，小刚给他3张后，自己的邮票张数就是小明邮票的2倍．小刚原来有邮票多少张．
<gadget id="calculator">6 * 2</gadget>
<output>12</output>

<gadget id="calculator">3 * 2</gadget>
<output>6</output>

<gadget id="calculator">3 + 12 + 6</gadget>
<output>21</output>

<result>21</result>