Multiply $ \dfrac{x+3}{x^2-16} $ by $ \dfrac{5x+20}{x+3} $ to get $ \dfrac{ 5 }{ x-4 } $.

Step 1: Cancel $ \color{blue}{ x+3 } $ in first and second fraction.

Step 2: Factor numerators and denominators.

Step 3: Cancel common factors.

Step 4: Multiply numerators and denominators.

Step 5: Simplify numerator and denominator.

$$ \begin{aligned} \frac{x+3}{x^2-16} \cdot \frac{5x+20}{x+3} & \xlongequal{\text{Step 1}} \frac{\color{blue}{1}}{x^2-16} \cdot \frac{5x+20}{\color{blue}{1}} = \\[1ex] & \xlongequal{\text{Step 2}} \frac{ 1 }{ \left( x-4 \right) \cdot \color{red}{ \left( x+4 \right) } } \cdot \frac{ 5 \cdot \color{red}{ \left( x+4 \right) } }{ 1 } = \\[1ex] & \xlongequal{\text{Step 3}} \frac{ 1 }{ x-4 } \cdot \frac{ 5 }{ 1 } \xlongequal{\text{Step 4}} \frac{ 1 \cdot 5 }{ \left( x-4 \right) \cdot 1 } \xlongequal{\text{Step 5}} \frac{ 5 }{ x-4 } \end{aligned} $$