One man's simple function is another man's nightmare
If you have a sum where dominates, i.e., , then . For example, .
It is customary to write n^2 for . It is even better to surround math with [tex] tags, e.g., [tex]f(n)=(2n +n^2)(n^3 +3n)[/tex] for .
Also, you are supposed to show some effort in solving a problem. At least write what you know and what your difficulty is.