Subtract $ \dfrac{3}{5} $ from $ \dfrac{5}{4x} $ to get $ \dfrac{ \color{purple}{ -12x+25 } }{ 20x }$.

To subtract raitonal expressions, both fractions must have the same denominator.
We can create a common denominator by multiplying the first fraction by $ \color{blue}{ 5 }$ and the second by $\color{blue}{ 4x }$.

$$ \begin{aligned} \frac{5}{4x} - \frac{3}{5} & = \frac{ 5 \cdot \color{blue}{ 5 }}{ 4x \cdot \color{blue}{ 5 }} - \frac{ 3 \cdot \color{blue}{ 4x }}{ 5 \cdot \color{blue}{ 4x }} = \\[1ex] &=\frac{ \color{purple}{ 25 } }{ 20x } - \frac{ \color{purple}{ 12x } }{ 20x }=\frac{ \color{purple}{ -12x+25 } }{ 20x } \end{aligned} $$