If 6 is the middle term of the proportion containing X and 9, then X=how much.
如果6是含X和9的比例的中项，则X=多少．
<gadget id="calculator">6 ** 2</gadget>
<output>36</output>

<gadget id="calculator">36 / 9</gadget>
<output>4</output>

<result>4</result>