The product of 5 and 9 minus 3 times a number is 2.1, find this number.
5与9的积减去一个数的3倍是2.1，求这个数．
<gadget id="calculator">5 * 9</gadget>
<output>45</output>

<gadget id="calculator">45 - 2.1</gadget>
<output>42.9</output>

<gadget id="calculator">42.9 / 3</gadget>
<output>14.3</output>

<result>14.3</result>