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

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