The perimeter of an isosceles trapezoid is 30 cm, its upper base is 3 cm long, and its lower base is 9 cm long. What is the waist length of this isosceles trapezoid?
等腰梯形的周长是30厘米，它的上底长3厘米，下底长9厘米，这个等腰梯形的腰长是多少厘米？
<gadget id="calculator">30 - 3 - 9</gadget>
<output>18</output>

<gadget id="calculator">18 / 2</gadget>
<output>9</output>

<result>9</result>