Let’s find horizontal asymptotes with help of the limits of this function at x->+oo and x->-oo
$$\lim_{x \to -\infty}\left(5 \sin{\left(5 x \right)} + 9 \sin{\left(9 x \right)}\right) = \left\langle -14, 14\right\rangle$$
Let's take the limitso,
equation of the horizontal asymptote on the left:
$$y = \left\langle -14, 14\right\rangle$$
$$\lim_{x \to \infty}\left(5 \sin{\left(5 x \right)} + 9 \sin{\left(9 x \right)}\right) = \left\langle -14, 14\right\rangle$$
Let's take the limitso,
equation of the horizontal asymptote on the right:
$$y = \left\langle -14, 14\right\rangle$$