Simplify the asymptotic expressions $O(n^2 + n) + \Omega (n^2 + n \log n)$
The expression that you write means what can we say about $f(n) + g(n)$ such that $f(n) \in O(n^2 + n)$ and $g(n) \in \Omega(n^2 + n \log n)$. First, we can rewrite the inputs to $f(n) \in O(n^2)$ and $g(n) \in \Omega(n^2)$. Hence, the we can say $f(n) + g(n) \in \Omega(n^2)$. 

However, we cannot say $f(n) + g(n) \in \Theta(n^2)$. As a counterexample, $f(n) = n$ and $g(n)  = n^3$. 

Anyhow, if you want to say there is a function $f(n)$ such that it is in $O(n^2 + n)$ and also in $\Omega(n^2 + n \log n)$, you're right. Although if $f(n) \in \Theta(n^2)$, it is in $\Omega(n^2)$ as well.