For $$15n^3+9n \in O\left(n^3\right)$$
I used the proof method to divide $f(n)$ by $g(n)$ to drive a constant of $24$ for all $n\:\ge \:1$.
However, I just realized the problem was asking to do it the direct way. I spent hours looking up the direct way for doing the big - o to find the positive real constant and non-negative integer, but can't find anything that isn't code.
Can someone help?