A rectangle with a length of 20dm and a width of 9dm is divided into a triangle and a trapezoid, the area difference is 18dm**2, find the upper base of the trapezoid.
一个长20dm，宽9dm的长方形分成一个三角形和一个梯形，面积差为18dm**2，求梯形的上底．
<gadget id="calculator">18 / 9</gadget>
<output>2</output>

<result>2</result>