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=  1 / ( 256^(1/4) )^3  =   1/ (4)^3 = 1/64  

$256^{\frac{-3}{4}}\\ =(\sqrt[4]{256})^{-3}\\ =\sqrt[4]{4^4}^{-3}\\ = 4^{-3}\\ =\dfrac{1}{4^3}\\ =\dfrac{1}{64}$