For horizontal asymptote - you need to investigate the value of y as x -> infinity. To do that, rearrange your equation
\(\displaystyle y = f(x)\)
in such a way that, it becomes
\(\displaystyle y = C + \frac{f_1(x)}{f_2(x)}\\)
where the degree of
\(\displaystyle f_1(x)\)
is lower than that of
\(\displaystyle f_2(x)\).
In that case your horizontal asymptote is
y = C