Multiply $5$ by $ \dfrac{x}{4x^3} $ to get $ \dfrac{ 5x }{ 4x^3 } $.

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.

$$ \begin{aligned} 5 \cdot \frac{x}{4x^3} & \xlongequal{\text{Step 1}} \frac{5}{\color{red}{1}} \cdot \frac{x}{4x^3} \xlongequal{\text{Step 2}} \frac{ 5 \cdot x }{ 1 \cdot 4x^3 } = \\[1ex] & \xlongequal{\text{Step 3}} \frac{ 5x }{ 4x^3 } \end{aligned} $$

Multiply $2$ by $ \dfrac{x}{4x+6} $ to get $ \dfrac{ 2x }{ 4x+6 } $.

Step 1: Write $ 2 $ as a fraction by putting $ \color{red}{1} $ in the denominator.

Step 2: Multiply numerators and denominators.

Step 3: Simplify numerator and denominator.

$$ \begin{aligned} 2 \cdot \frac{x}{4x+6} & \xlongequal{\text{Step 1}} \frac{2}{\color{red}{1}} \cdot \frac{x}{4x+6} \xlongequal{\text{Step 2}} \frac{ 2 \cdot x }{ 1 \cdot \left( 4x+6 \right) } = \\[1ex] & \xlongequal{\text{Step 3}} \frac{ 2x }{ 4x+6 } \end{aligned} $$

Subtract $ \dfrac{2x}{4x+6} $ from $ \dfrac{5x}{4x^3} $ to get $ \dfrac{ \color{purple}{ -8x^4+20x^2+30x } }{ 16x^4+24x^3 }$.

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}{ 4x+6 }$ and the second by $\color{blue}{ 4x^3 }$.

$$ \begin{aligned} \frac{5x}{4x^3} - \frac{2x}{4x+6} & = \frac{ 5x \cdot \color{blue}{ \left( 4x+6 \right) }}{ 4x^3 \cdot \color{blue}{ \left( 4x+6 \right) }} - \frac{ 2x \cdot \color{blue}{ 4x^3 }}{ \left( 4x+6 \right) \cdot \color{blue}{ 4x^3 }} = \\[1ex] &=\frac{ \color{purple}{ 20x^2+30x } }{ 16x^4+24x^3 } - \frac{ \color{purple}{ 8x^4 } }{ 16x^4+24x^3 }=\frac{ \color{purple}{ -8x^4+20x^2+30x } }{ 16x^4+24x^3 } \end{aligned} $$