A trapezoidal piece of iron, the height is 6 cm, the upper base is 6 cm, and the lower base is 12 cm. Cut out the largest triangle from the trapezoidal iron plate. What is the remaining area in square centimeters?
一块梯形铁片，高是6厘米，上底是6厘米，下底是12厘米，在梯形铁片上剪下一个最大的三角形，所剩的面积是多少平方厘米．
<gadget id="calculator">6 + 12</gadget>
<output>18</output>

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

<gadget id="calculator">18 * 3</gadget>
<output>54</output>

<gadget id="calculator">12 * 3</gadget>
<output>36</output>

<gadget id="calculator">54 - 36</gadget>
<output>18</output>

<result>18</result>