Multiply $ \color{blue}{16} $ by $ \left( 5x+20\right) $ $$ \color{blue}{16} \cdot \left( 5x+20\right) = 80x+320 $$

Subtract $80x+320$ from $ \dfrac{x+3}{x^2} $ to get $ \dfrac{ \color{purple}{ -80x^3-320x^2+x+3 } }{ x^2 }$.

Step 1: Write $ 80x+320 $ 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 $\color{blue}{x^2}$.

$$ \begin{aligned} \frac{x+3}{x^2} -80x+320 & \xlongequal{\text{Step 1}} \frac{x+3}{x^2} - \frac{80x+320}{\color{red}{1}} = \frac{ x+3 }{ x^2 } - \frac{ \left( 80x+320 \right) \cdot \color{blue}{ x^2 }}{ 1 \cdot \color{blue}{ x^2 }} = \\[1ex] & \xlongequal{\text{Step 2}} \frac{ \color{purple}{ x+3 } }{ x^2 } - \frac{ \color{purple}{ 80x^3+320x^2 } }{ x^2 }=\frac{ \color{purple}{ x+3 - \left( 80x^3+320x^2 \right) } }{ x^2 } = \\[1ex] &=\frac{ \color{purple}{ -80x^3-320x^2+x+3 } }{ x^2 } \end{aligned} $$