If xm=16, xn=128, find the value of x2m-n.
若xm=16，xn=128，求x2m-n的值．
<gadget id="calculator">16 ** 2</gadget>
<output>256</output>

<gadget id="calculator">256 / 128</gadget>
<output>2</output>

<result>2</result>