Asymptotics of a function that decreases as n increases
If you use suitable definitions, you'll find that your function is in


$O(2^n)$,
$O(n^4)$,
$o(1)$ and
$\Omega(-n^2)$,


among many others. It's likely that the question (implicitly) asks for sharper bounds; I'll leave that for you to figure out.