$\begin{align*} & \;\; \;\; \; F(x) = O(G(x)) \\ &\Rightarrow |F(x)| \leq {\bf c}|G(x)| \;\; \text{ for all } x \geq x_0 \text{ and } x_0,c > 0 \end{align*}$
In this QS :
$\begin{align*} &F(x) = n\log_2 n \\ &G(x) = n^2 \\ &\text{We can easily observe that }\;\; n\log_2 n = O(n^2) \;\; \text{ for c = 1 and }x_0 = 2 \end{align*}$