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$\displaystyle 2^{-n} \left ( 2^n - 2^{1 + n} \right )$
First, note that multiplication is distributive over addition, so this is:
$\displaystyle 2^{-n} \left ( 2^n - 2^{1 + n} \right ) = 2^{-n} \cdot 2^n - 2^{-n} \cdot 2^{1 + n}$
Now use the property that
$\displaystyle a^x \cdot a^y = a^{x + y}$
So:
$\displaystyle = 2^{-n + n} - 2^{-n + 1 + n} = 2^0 - 2^1$
$\displaystyle = 1 - 2 = -1$
-Dan