Multiply $3$ by $ \dfrac{b}{b} $ to get $ \dfrac{ 3b }{ b } $.

Step 1: Write $ 3 $ as a fraction by putting $ \color{red}{1} $ in the denominator.

Step 2: Multiply numerators and denominators.

Step 3: Simplify numerator and denominator.

$$ \begin{aligned} 3 \cdot \frac{b}{b} & \xlongequal{\text{Step 1}} \frac{3}{\color{red}{1}} \cdot \frac{b}{b} \xlongequal{\text{Step 2}} \frac{ 3 \cdot b }{ 1 \cdot b } \xlongequal{\text{Step 3}} \frac{ 3b }{ b } \end{aligned} $$

Subtract $2$ from $ \dfrac{3b}{b} $ to get $ \dfrac{ \color{purple}{ b } }{ b }$.

Step 1: Write $ 2 $ 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 second fraction by $\color{blue}{b}$.

$$ \begin{aligned} \frac{3b}{b} -2 & \xlongequal{\text{Step 1}} \frac{3b}{b} - \frac{2}{\color{red}{1}} = \frac{ 3b }{ b } - \frac{ 2 \cdot \color{blue}{ b }}{ 1 \cdot \color{blue}{ b }} = \\[1ex] & \xlongequal{\text{Step 2}} \frac{ \color{purple}{ 3b } }{ b } - \frac{ \color{purple}{ 2b } }{ b }=\frac{ \color{purple}{ b } }{ b } \end{aligned} $$