It is known that the area of the trapezoid is 105 square centimeters, the height is 7 centimeters, and the upper base is 12 cm. Find the length of the lower base. (column calculation)
已知梯形的面积是105平方厘米，高是7厘米，上底是12厘米，求下底的长．(列式计算)
<gadget id="calculator">2 / 7</gadget>
<output>2/7 = around 0.285714</output>

<gadget id="calculator">105 * (2/7)</gadget>
<output>30</output>

<gadget id="calculator">30 - 12</gadget>
<output>18</output>

<result>18</result>