I need to prove or disprove $n\log_2(n)-3n-18 = \Omega(n)$ and get appropriate constants so that the lower bound limit definition is fulfilled.
My attempt In order to prove that relation, I use the notion of limit:
$$\lim_{n \to \infty} \frac{n\log_2(n)-3n-18}{n} = \left[\frac{\infty}{\infty}\right]$$
so applying L'Hôpital theorem and deriving numerator and denominator, I get
$$\lim_{n\to \infty} \frac{\log_2n+\dfrac{n}{n\log2}-3}{1} = \infty $$
so at this point I know that $n\log_2(n)-3n-18 = \omega(n)$... and that's where I get stuck and basically try to guess, so I am wondering if I can calculate the values I need: how do I select $c$ and $n_0$ so that $\exists c > 0$, $n_0 > 0$ such that $\forall n \geq n_0$, $f(n) \geq c\cdot g(n) \geq 0$?
You may want to show that for $n$ "big enough" you have $n \log_2 n - 3n -18 \geq n$. Note that for $n\geq 36$, we certainly have $\frac{1}{2}n \log_2 n \geq 18$, so it suffices to find $n_0\geq 36$ such that $$ \frac{1}{2}n\log_2 n - 3n \geq n $$ i.e. $$ n\log_2 n \geq 8n. $$ You should be able to do so easily.
Once you have this $n_0\geq 36$, you get that $$ n \log_2 n - 3n -18 \geq \frac{1}{2}n\log_2 n + 18 - 3n -18 = \frac{1}{2}n\log_2 n -3n \geq n $$ for all $n\geq n_0$, which shows what you want we constant $c=1$.
Note however that you then show $f(n)=\Omega(n)$ (with a tedious approach), while your direct limit argument immediately establishes the stronger $f(n)=\omega(n)$. The "find a constant and $n_0$ approach" is definitely not the simplest or most effective...