Add $ \dfrac{3}{8} $ and $ \dfrac{3}{3n+4} $ to get $ \dfrac{ \color{purple}{ 9n+36 } }{ 24n+32 }$.

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}{ 3n+4 }$ and the second by $\color{blue}{ 8 }$.

$$ \begin{aligned} \frac{3}{8} + \frac{3}{3n+4} & = \frac{ 3 \cdot \color{blue}{ \left( 3n+4 \right) }}{ 8 \cdot \color{blue}{ \left( 3n+4 \right) }} + \frac{ 3 \cdot \color{blue}{ 8 }}{ \left( 3n+4 \right) \cdot \color{blue}{ 8 }} = \\[1ex] &=\frac{ \color{purple}{ 9n+12 } }{ 24n+32 } + \frac{ \color{purple}{ 24 } }{ 24n+32 }=\frac{ \color{purple}{ 9n+36 } }{ 24n+32 } \end{aligned} $$