We have indeterminateness of type
0/0,
i.e. limit for the numerator is
$$\lim_{x \to 0^+} \operatorname{asin}{\left(2 x \right)} = 0$$
and limit for the denominator is
$$\lim_{x \to 0^+}\left(8 x\right) = 0$$
Let's take derivatives of the numerator and denominator until we eliminate indeterninateness.
$$\lim_{x \to 0^+}\left(\frac{\operatorname{asin}{\left(2 x \right)}}{8 x}\right)$$
=
Let's transform the function under the limit a few
$$\lim_{x \to 0^+}\left(\frac{\operatorname{asin}{\left(2 x \right)}}{8 x}\right)$$
=
$$\lim_{x \to 0^+}\left(\frac{\frac{d}{d x} \operatorname{asin}{\left(2 x \right)}}{\frac{d}{d x} 8 x}\right)$$
=
$$\lim_{x \to 0^+}\left(\frac{1}{4 \sqrt{1 - 4 x^{2}}}\right)$$
=
$$\lim_{x \to 0^+} \frac{1}{4}$$
=
$$\lim_{x \to 0^+} \frac{1}{4}$$
=
$$\frac{1}{4}$$
It can be seen that we have applied Lopital's rule (we have taken derivatives with respect to the numerator and denominator) 1 time(s)