Inclined asymptote can be found by calculating the limit of log(1/8)*x, divided by x at x->+oo and x ->-oo
$$\lim_{x \to -\infty} \log{\left(\frac{1}{8} \right)} = - \log{\left(8 \right)}$$
Let's take the limitso,
inclined asymptote equation on the left:
$$y = - x \log{\left(8 \right)}$$
$$\lim_{x \to \infty} \log{\left(\frac{1}{8} \right)} = - \log{\left(8 \right)}$$
Let's take the limitso,
inclined asymptote equation on the right:
$$y = - x \log{\left(8 \right)}$$