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For which of the following values of $x$ does:

$${ \log }_{ x }\frac { 1 }{ 8 } =-\frac { 3 }{ 4 }$$

$ -16 $

$ -\cfrac { 1 }{ 16 } $

$ \cfrac { 1 }{ 16 } $

$ 8 $

$ 16 $