\[\frac{5 + 4n + 1}{2} \cdot n = 324\]
\[(3 + 2n)n = 324\]
\[2n^{2} + 3n - 324 = 0\ \ \ \ \ \ \ \]
\[D = 9 + 2592 = 2601\]
\[n_{1} = \frac{- 3 + 51}{4} = 12,\ \ \ \ \]
\[n_{2} = \frac{- 3 - 51}{4} < 0 \Longrightarrow \ не\ \]
\[удовлетворяет.\]
\[Ответ:\ n = 12.\]