Add $ \dfrac{7}{8x} $ and $ \dfrac{5}{2x} $ to get $ \dfrac{ \color{purple}{ 54x } }{ 16x^2 }$.

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}{ 2x }$ and the second by $\color{blue}{ 8x }$.

$$ \begin{aligned} \frac{7}{8x} + \frac{5}{2x} & = \frac{ 7 \cdot \color{blue}{ 2x }}{ 8x \cdot \color{blue}{ 2x }} + \frac{ 5 \cdot \color{blue}{ 8x }}{ 2x \cdot \color{blue}{ 8x }} = \\[1ex] &=\frac{ \color{purple}{ 14x } }{ 16x^2 } + \frac{ \color{purple}{ 40x } }{ 16x^2 }=\frac{ \color{purple}{ 54x } }{ 16x^2 } \end{aligned} $$