A trapezoid, the sum of the upper and lower bases is 28 cm, and the height is 6 cm, what is its area in square centimeters?
一个梯形，上、下底的和是28厘米，高是6厘米，它的面积是多少平方厘米．
<gadget id="calculator">6 / 2</gadget>
<output>3</output>

<gadget id="calculator">28 * 3</gadget>
<output>84</output>

<result>84</result>