Subtract $ \dfrac{4}{x} $ from $ \dfrac{4}{5} $ to get $ \dfrac{ \color{purple}{ 4x-20 } }{ 5x }$.

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}{ x }$ and the second by $\color{blue}{ 5 }$.

$$ \begin{aligned} \frac{4}{5} - \frac{4}{x} & = \frac{ 4 \cdot \color{blue}{ x }}{ 5 \cdot \color{blue}{ x }} - \frac{ 4 \cdot \color{blue}{ 5 }}{ x \cdot \color{blue}{ 5 }} = \\[1ex] &=\frac{ \color{purple}{ 4x } }{ 5x } - \frac{ \color{purple}{ 20 } }{ 5x }=\frac{ \color{purple}{ 4x-20 } }{ 5x } \end{aligned} $$

Divide $x^2$ by $ \dfrac{4x-20}{5x} $ to get $ \dfrac{ 5x^3 }{ 4x-20 } $.

Step 1: To divide rational expressions, multiply the first fraction by the reciprocal of the second fraction.

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

Step 3: Multiply numerators and denominators.

Step 4: Simplify numerator and denominator.

$$ \begin{aligned} \frac{x^2}{ \frac{\color{blue}{4x-20}}{\color{blue}{5x}} } & \xlongequal{\text{Step 1}} x^2 \cdot \frac{\color{blue}{5x}}{\color{blue}{4x-20}} = \\[1ex] & \xlongequal{\text{Step 2}} \frac{x^2}{\color{red}{1}} \cdot \frac{5x}{4x-20} \xlongequal{\text{Step 3}} \frac{ x^2 \cdot 5x }{ 1 \cdot \left( 4x-20 \right) } = \\[1ex] & \xlongequal{\text{Step 4}} \frac{ 5x^3 }{ 4x-20 } \end{aligned} $$