Subtract $ \dfrac{1}{x} $ from $ \dfrac{1}{3} $ to get $ \dfrac{ \color{purple}{ x-3 } }{ 3x }$.

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 $\color{blue}{ 3 }$.

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