The area of the trapezoid is 63 square meters and the height is 7 meters. It is known that the upper base is 4 meters shorter than the lower base. Find the height of the lower base.
梯形的面积是63平方米，高是7米，已知上底比下底少4米，求下底的高度．
<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>