It is known that the upper base of a trapezoid is 6 cm, the lower base is twice as large as the upper base, and the height is 4 cm less than the lower base. What is the area of the trapezoid in square centimeters?
已知一个梯形的上底是6厘米，下底是上底的2倍，高比下底少4厘米，这个梯形的面积是多少平方厘米．
<gadget id="calculator">6 * 2</gadget>
<output>12</output>

<gadget id="calculator">6 + 12</gadget>
<output>18</output>

<gadget id="calculator">12 - 4</gadget>
<output>8</output>

<gadget id="calculator">8 / 2</gadget>
<output>4</output>

<gadget id="calculator">18 * 4</gadget>
<output>72</output>

<result>72</result>