Figuring out the lowest Big-O for $\ 7n^2$ for example, is straight-forward by finding witnesses C and k such as $\ n > 7 $ therefore $\ 7n^2 < n^3 $ so k = 7 and C = 1. So $\ 7n^2 $ has Big-O($\ n^3$)
But, how do I find the lowest Big-O for something like $\ 3^{n^3}$?