Add $ \dfrac{8}{9x} $ and $ \dfrac{1}{9x^2} $ to get $ \dfrac{ \color{purple}{ 72x^2+9x } }{ 81x^3 }$.

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^2 }$ and the second by $\color{blue}{ 9x }$.

$$ \begin{aligned} \frac{8}{9x} + \frac{1}{9x^2} & = \frac{ 8 \cdot \color{blue}{ 9x^2 }}{ 9x \cdot \color{blue}{ 9x^2 }} + \frac{ 1 \cdot \color{blue}{ 9x }}{ 9x^2 \cdot \color{blue}{ 9x }} = \\[1ex] &=\frac{ \color{purple}{ 72x^2 } }{ 81x^3 } + \frac{ \color{purple}{ 9x } }{ 81x^3 }=\frac{ \color{purple}{ 72x^2+9x } }{ 81x^3 } \end{aligned} $$