From 2 sticks of 3 cm and 2 sticks of 9 cm, choose 3 sticks to form a triangle. What is the perimeter of this triangle in cm?
从2根3cm，2根9cm的小棒中，挑选3根围成一个三角形，这个三角形的周长是多少cm．
<gadget id="calculator">3 + 9 + 9</gadget>
<output>21</output>

<result>21</result>