If 3x-2x=7, then 3x-21=how much.
若3x-2x=7，则3x-21=多少．
<gadget id="calculator">3 - 2</gadget>
<output>1</output>

<gadget id="calculator">7 / 1</gadget>
<output>7</output>

<gadget id="calculator">3 * 7</gadget>
<output>21</output>

<gadget id="calculator">21 - 21</gadget>
<output>0</output>

<result>0</result>