The perimeter of a right-angled trapezoid is 42 cm. The ratio of the sum of the lengths of the upper and lower bases to the sum of the lengths of the two waists is 2:1, and the ratio of one waist to the other is 3:4. What is the area in square centimeters.
一个直角梯形的周长是42厘米，上、下底的长度之和与两条腰的长度之和的比是2：1，一条腰与另一条腰的比是3：4，这个直角梯形的面积是多少平方厘米．
<gadget id="calculator">1 + 2</gadget>
<output>3</output>

<gadget id="calculator">2 / 3</gadget>
<output>2/3 = around 0.666667</output>

<gadget id="calculator">42 * (2/3)</gadget>
<output>28</output>

<gadget id="calculator">1 / 3</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">42 * (1/3)</gadget>
<output>14</output>

<gadget id="calculator">3 + 4</gadget>
<output>7</output>

<gadget id="calculator">3 / 7</gadget>
<output>3/7 = around 0.428571</output>

<gadget id="calculator">14 * (3/7)</gadget>
<output>6</output>

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

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

<result>84</result>