I know that by the assumption that $f ∈ Θ(g)$ we know that there exist constants $c_0$ and $c_1$ in $\mathbb R^+$, and there exists $n_0 ∈ \mathbb N$ such that:
$c_0 \cdot g(n)\le f(n)\le c_1 \cdot g(n),$
but this is then where I get stuck, can anyone help me out?