rectangle with a length of 7cm and a width of 10cm is divided into a triangle and a trapezoid by a line segment. The difference between their areas is 28cm**2. What is the length of the upper base of the trapezoid in cm?
一个长方形长7cm、宽10cm，被一条线段分割成了一个三角形和一个梯形，它们的面积相差28cm**2，梯形的上底长是多少cm．
<gadget id="calculator">7 / 2</gadget>
<output>7/2 = around 3.5</output>

<gadget id="calculator">10 * (7/2)</gadget>
<output>35</output>

<gadget id="calculator">28 - 35</gadget>
<output>-7</output>

<gadget id="calculator">35 + (-7)</gadget>
<output>28</output>

<gadget id="calculator">(7/2) + (7/2)</gadget>
<output>7</output>

<gadget id="calculator">28 / 7</gadget>
<output>4</output>

<result>4</result>