Ah, thanks! My reasoning didn't really make much sense looking back on it now.
Let's say that, for negative k, we use ##k = -n## where n > 0, so we have ##(x-n)^3 = x^3 + 8x^2 - 6x + 8##
Importantly, ##8x^2 - 6x + 8 > 0## for all positive x.
However, upon expanding, and canceling x^3 on both...