I am looking to find the lower bound for a function $f(n) = \frac{n^5}{n^3 + 3n}$ where $g(n) = n^2$. I am looking to prove that $f(n)$ is $\theta(n^2)$ and have already proved that $f(n) = O(n^2)$ but cannot find the lower bound for this function and prove $f(n)=\Omega(n^2)$.