Add $ \dfrac{5}{3x} $ and $ \dfrac{1}{9x} $ to get $ \dfrac{ \color{purple}{ 48x } }{ 27x^2 }$.

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}{ 9x }$ and the second by $\color{blue}{ 3x }$.

$$ \begin{aligned} \frac{5}{3x} + \frac{1}{9x} & = \frac{ 5 \cdot \color{blue}{ 9x }}{ 3x \cdot \color{blue}{ 9x }} + \frac{ 1 \cdot \color{blue}{ 3x }}{ 9x \cdot \color{blue}{ 3x }} = \\[1ex] &=\frac{ \color{purple}{ 45x } }{ 27x^2 } + \frac{ \color{purple}{ 3x } }{ 27x^2 }=\frac{ \color{purple}{ 48x } }{ 27x^2 } \end{aligned} $$