Say for example I say that:
$ 2n^2 + n - 8 \quad\text{is}\quad O(n^3) $
To prove this I must find a constant $c$ and a point $n_0$ for which $n^3$ is an upper bound of the equation.
This is easy to see because $n^3$ will always be larger than $2n^2$ (dominant term) for any constant $c$. How do I find $n_0$?