Multiply $5$ by $ \dfrac{x}{2x+6} $ to get $ \dfrac{ 5x }{ 2x+6 } $.

Step 1: Write $ 5 $ as a fraction by putting $ \color{red}{1} $ in the denominator.

Step 2: Multiply numerators and denominators.

Step 3: Simplify numerator and denominator.

$$ \begin{aligned} 5 \cdot \frac{x}{2x+6} & \xlongequal{\text{Step 1}} \frac{5}{\color{red}{1}} \cdot \frac{x}{2x+6} \xlongequal{\text{Step 2}} \frac{ 5 \cdot x }{ 1 \cdot \left( 2x+6 \right) } = \\[1ex] & \xlongequal{\text{Step 3}} \frac{ 5x }{ 2x+6 } \end{aligned} $$

Subtract $ \dfrac{x}{6x+18} $ from $ \dfrac{5x}{2x+6} $ to get $ \dfrac{ \color{purple}{ 14x } }{ 6x+18 }$.

To subtract raitonal expressions, both fractions must have the same denominator.
We can create a common denominator by multiplying the first fraction by $\color{blue}{ 3 }$.

$$ \begin{aligned} \frac{5x}{2x+6} - \frac{x}{6x+18} & = \frac{ 5x \cdot \color{blue}{ 3 }}{ \left( 2x+6 \right) \cdot \color{blue}{ 3 }} - \frac{ x }{ 6x+18 } = \\[1ex] &=\frac{ \color{purple}{ 15x } }{ 6x+18 } - \frac{ \color{purple}{ x } }{ 6x+18 }=\frac{ \color{purple}{ 14x } }{ 6x+18 } \end{aligned} $$