There is a new operator "○" which makes the following equations true: 5○3=12, 3○5=4, 8○4=20, 3○4=5. Calculate the value of 7○2 according to this law.
有一个新运算符"○"使下列等式成立：5○3=12，3○5=4，8○4=20，3○4=5．按此规律计算7○2的值．
<gadget id="calculator">3 * 7</gadget>
<output>21</output>

<gadget id="calculator">21 - 2</gadget>
<output>19</output>

<result>19</result>