Subtract $ \dfrac{2}{x} $ from $ \dfrac{4}{3x} $ to get $ \dfrac{ \color{purple}{ -2x } }{ 3x^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}{ x }$ and the second by $\color{blue}{ 3x }$.

$$ \begin{aligned} \frac{4}{3x} - \frac{2}{x} & = \frac{ 4 \cdot \color{blue}{ x }}{ 3x \cdot \color{blue}{ x }} - \frac{ 2 \cdot \color{blue}{ 3x }}{ x \cdot \color{blue}{ 3x }} = \\[1ex] &=\frac{ \color{purple}{ 4x } }{ 3x^2 } - \frac{ \color{purple}{ 6x } }{ 3x^2 }=\frac{ \color{purple}{ -2x } }{ 3x^2 } \end{aligned} $$