Precalculus (6th Edition) Blitzer

The equation of the horizontal asymptote for the function $f\left( x \right)$ is $\underline{y=0}$.
The vertical asymptote is a horizontal line which the function approaches but does not touch. It is clear from the graph that the function approaches the line $y=0$ line but does not touch it. Therefore, the equation of the horizontal asymptote is $y=0$.