Add $ \dfrac{1}{n} $ and $ \dfrac{7n+6+2n^3+3n^2}{6n^3} $ to get $ \dfrac{ \color{purple}{ 2n^4+9n^3+7n^2+6n } }{ 6n^4 }$.

To add raitonal expressions, both fractions must have the same denominator.
We can create a common denominator by multiplying the first fraction by $ \color{blue}{ 6n^3 }$ and the second by $\color{blue}{ n }$.

$$ \begin{aligned} \frac{1}{n} + \frac{7n+6+2n^3+3n^2}{6n^3} & = \frac{ 1 \cdot \color{blue}{ 6n^3 }}{ n \cdot \color{blue}{ 6n^3 }} + \frac{ \left( 7n+6+2n^3+3n^2 \right) \cdot \color{blue}{ n }}{ 6n^3 \cdot \color{blue}{ n }} = \\[1ex] &=\frac{ \color{purple}{ 6n^3 } }{ 6n^4 } + \frac{ \color{purple}{ 7n^2+6n+2n^4+3n^3 } }{ 6n^4 }=\frac{ \color{purple}{ 2n^4+9n^3+7n^2+6n } }{ 6n^4 } \end{aligned} $$