Subtract $ \dfrac{4}{5x} $ from $ \dfrac{7}{3x} $ to get $ \dfrac{ \color{purple}{ 23x } }{ 15x^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}{ 5x }$ and the second by $\color{blue}{ 3x }$.

$$ \begin{aligned} \frac{7}{3x} - \frac{4}{5x} & = \frac{ 7 \cdot \color{blue}{ 5x }}{ 3x \cdot \color{blue}{ 5x }} - \frac{ 4 \cdot \color{blue}{ 3x }}{ 5x \cdot \color{blue}{ 3x }} = \\[1ex] &=\frac{ \color{purple}{ 35x } }{ 15x^2 } - \frac{ \color{purple}{ 12x } }{ 15x^2 }=\frac{ \color{purple}{ 23x } }{ 15x^2 } \end{aligned} $$