Add $ \dfrac{7}{4x} $ and $ \dfrac{5}{4x^2} $ to get $ \dfrac{ \color{purple}{ 28x^2+20x } }{ 16x^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}{ 4x^2 }$ and the second by $\color{blue}{ 4x }$.

$$ \begin{aligned} \frac{7}{4x} + \frac{5}{4x^2} & = \frac{ 7 \cdot \color{blue}{ 4x^2 }}{ 4x \cdot \color{blue}{ 4x^2 }} + \frac{ 5 \cdot \color{blue}{ 4x }}{ 4x^2 \cdot \color{blue}{ 4x }} = \\[1ex] &=\frac{ \color{purple}{ 28x^2 } }{ 16x^3 } + \frac{ \color{purple}{ 20x } }{ 16x^3 }=\frac{ \color{purple}{ 28x^2+20x } }{ 16x^3 } \end{aligned} $$