hi , I learned in class that if an Oblique asymptote is present, then the degree of the numerator should be exactly 1 more than the denominator's. However this does not apply to $\displaystyle f(x) = (x^3-16x)/(-4x^2+4x+24)$ how come?

I mean I looked at its appropriate graph, and I could not see an OA, According to my calculations it is: $\displaystyle y= 1/4 x + 1/4$.