Subtract $ \dfrac{4}{3x} $ from $ \dfrac{5}{7x} $ to get $ \dfrac{ \color{purple}{ -13x } }{ 21x^2 }$.

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}{ 3x }$ and the second by $\color{blue}{ 7x }$.

$$ \begin{aligned} \frac{5}{7x} - \frac{4}{3x} & = \frac{ 5 \cdot \color{blue}{ 3x }}{ 7x \cdot \color{blue}{ 3x }} - \frac{ 4 \cdot \color{blue}{ 7x }}{ 3x \cdot \color{blue}{ 7x }} = \\[1ex] &=\frac{ \color{purple}{ 15x } }{ 21x^2 } - \frac{ \color{purple}{ 28x } }{ 21x^2 }=\frac{ \color{purple}{ -13x } }{ 21x^2 } \end{aligned} $$