Add $ \dfrac{3}{5x^2} $ and $ \dfrac{4}{10x} $ to get $ \dfrac{ \color{purple}{ 20x^2+30x } }{ 50x^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}{ 10x }$ and the second by $\color{blue}{ 5x^2 }$.

$$ \begin{aligned} \frac{3}{5x^2} + \frac{4}{10x} & = \frac{ 3 \cdot \color{blue}{ 10x }}{ 5x^2 \cdot \color{blue}{ 10x }} + \frac{ 4 \cdot \color{blue}{ 5x^2 }}{ 10x \cdot \color{blue}{ 5x^2 }} = \\[1ex] &=\frac{ \color{purple}{ 30x } }{ 50x^3 } + \frac{ \color{purple}{ 20x^2 } }{ 50x^3 }=\frac{ \color{purple}{ 20x^2+30x } }{ 50x^3 } \end{aligned} $$