The perimeter of an isosceles trapezoid is 36 cm, its upper base is 9 cm, and its waist is 6 cm. What is the length of the base of this isosceles trapezoid?
一个等腰梯形的周长是36厘米，它的上底是9厘米，腰长是6厘米．这个等腰梯形的下底长多少厘米．
<gadget id="calculator">6 * 2</gadget>
<output>12</output>

<gadget id="calculator">36 - 9 - 12</gadget>
<output>15</output>

<result>15</result>