When a=4, b=5, c=6, what is the value of bc-ac.
当a=4，b=5，c=6时，bc-ac的值是多少．
<gadget id="calculator">5 * 6</gadget>
<output>30</output>

<gadget id="calculator">4 * 6</gadget>
<output>24</output>

<gadget id="calculator">30 - 24</gadget>
<output>6</output>

<result>6</result>