Add $ \dfrac{2}{3x} $ and $ \dfrac{4}{x^2} $ to get $ \dfrac{ \color{purple}{ 2x^2+12x } }{ 3x^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}{ x^2 }$ and the second by $\color{blue}{ 3x }$.

$$ \begin{aligned} \frac{2}{3x} + \frac{4}{x^2} & = \frac{ 2 \cdot \color{blue}{ x^2 }}{ 3x \cdot \color{blue}{ x^2 }} + \frac{ 4 \cdot \color{blue}{ 3x }}{ x^2 \cdot \color{blue}{ 3x }} = \\[1ex] &=\frac{ \color{purple}{ 2x^2 } }{ 3x^3 } + \frac{ \color{purple}{ 12x } }{ 3x^3 }=\frac{ \color{purple}{ 2x^2+12x } }{ 3x^3 } \end{aligned} $$