Multiply $4$ by $ \dfrac{b}{12b-18} $ to get $ \dfrac{ 4b }{ 12b-18 } $.

Step 1: Write $ 4 $ as a fraction by putting $ \color{red}{1} $ in the denominator.

Step 2: Multiply numerators and denominators.

Step 3: Simplify numerator and denominator.

$$ \begin{aligned} 4 \cdot \frac{b}{12b-18} & \xlongequal{\text{Step 1}} \frac{4}{\color{red}{1}} \cdot \frac{b}{12b-18} \xlongequal{\text{Step 2}} \frac{ 4 \cdot b }{ 1 \cdot \left( 12b-18 \right) } = \\[1ex] & \xlongequal{\text{Step 3}} \frac{ 4b }{ 12b-18 } \end{aligned} $$

Add $ \dfrac{b}{6b-9} $ and $ \dfrac{4b}{12b-18} $ to get $ \dfrac{ \color{purple}{ 6b } }{ 12b-18 }$.

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

$$ \begin{aligned} \frac{b}{6b-9} + \frac{4b}{12b-18} & = \frac{ b \cdot \color{blue}{ 2 }}{ \left( 6b-9 \right) \cdot \color{blue}{ 2 }} + \frac{ 4b }{ 12b-18 } = \\[1ex] &=\frac{ \color{purple}{ 2b } }{ 12b-18 } + \frac{ \color{purple}{ 4b } }{ 12b-18 }=\frac{ \color{purple}{ 6b } }{ 12b-18 } \end{aligned} $$