mimendo: setting f(x) = 0 will only find the x-intercept.
To find the horizontal asymptote, I would just plug inifinity for x.
the top would become -infinity, and the bottom would be +infinity, and i Know you can't devide infinity/infinity (techincally, it's infinity) but, it would end up being -1.
horz. asym. = -1
another way is to find the highest powers of x for the top and bottom and divide accordingly.
For here, we have -x^1 on top, and x^1 on top, so take out anything that's not the highest power of x and that leaves you with -x^1/x^1 = -1.