Subtract $ \dfrac{3}{2} $ from $ 5b+4 $ to get $ \dfrac{ \color{purple}{ 10b+5 } }{ 2 }$.

Step 1: Write $ 5b+4 $ 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 $\color{blue}{ 2 }$.

$$ \begin{aligned} 5b+4- \frac{3}{2} & \xlongequal{\text{Step 1}} \frac{5b+4}{\color{red}{1}} - \frac{3}{2} = \frac{ \left( 5b+4 \right) \cdot \color{blue}{ 2 }}{ 1 \cdot \color{blue}{ 2 }} - \frac{ 3 }{ 2 } = \\[1ex] & \xlongequal{\text{Step 2}} \frac{ \color{purple}{ 10b+8 } }{ 2 } - \frac{ \color{purple}{ 3 } }{ 2 }=\frac{ \color{purple}{ 10b+5 } }{ 2 } \end{aligned} $$