Subtract $ \dfrac{4}{u} $ from $ 6 $ to get $ \dfrac{ \color{purple}{ 6u-4 } }{ u }$.

Step 1: Write $ 6 $ as a fraction by putting $ \color{red}{ 1 } $ in the denominator.

Step 2: To subtract raitonal expressions, both fractions must have the same denominator.
We can create a common denominator by multiplying the first fraction by $\color{blue}{ u }$.

$$ \begin{aligned} 6- \frac{4}{u} & \xlongequal{\text{Step 1}} \frac{6}{\color{red}{1}} - \frac{4}{u} = \frac{ 6 \cdot \color{blue}{ u }}{ 1 \cdot \color{blue}{ u }} - \frac{ 4 }{ u } = \\[1ex] & \xlongequal{\text{Step 2}} \frac{ \color{purple}{ 6u } }{ u } - \frac{ \color{purple}{ 4 } }{ u }=\frac{ \color{purple}{ 6u-4 } }{ u } \end{aligned} $$