The area of the trapezoid is 63m**2, the height is 7m, the upper base is 4m less than the lower base, what is the lower base in m?
梯形的面积是63m**2，高是7m，上底比下底少4m，下底是多少m．
<gadget id="calculator">63 * 2</gadget>
<output>126</output>

<gadget id="calculator">126 / 7</gadget>
<output>18</output>

<gadget id="calculator">18 + 4</gadget>
<output>22</output>

<gadget id="calculator">22 / 2</gadget>
<output>11</output>

<result>11</result>