Solution (source code)

= Solution

For $n\ge1$, $\log3<2$ implies $3^{\log n}=n^{\log3}\le n^2$, and also $n\le n^2$. Consequently
$$
0\le(n+3^{\log n})^2\le(2n^2)^2=4n^4.
$$
The definition of <big O notation> holds with $C=4,N=1$:
$$
\boxed{(n+3^{\log n})^2=O(n^4).}
$$