$x=\frac{1}{8} \to 64(\frac{1}{8})^3-48(\frac{1}{8})^2+12(\frac{1}{8})-1=\frac{1}{8}-\frac{6}{8}+\frac{12}{8}-\frac{8}{8}=\frac{1-6+12-8}{8}=\frac{-1}{8}$