Subtract $8a$ from $ \dfrac{4a^2-36}{24} $ to get $ \dfrac{ \color{purple}{ 4a^2-192a-36 } }{ 24 }$.

Step 1: Write $ 8a $ 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 $\color{blue}{24}$.

$$ \begin{aligned} \frac{4a^2-36}{24} -8a & \xlongequal{\text{Step 1}} \frac{4a^2-36}{24} - \frac{8a}{\color{red}{1}} = \frac{ 4a^2-36 }{ 24 } - \frac{ 8a \cdot \color{blue}{ 24 }}{ 1 \cdot \color{blue}{ 24 }} = \\[1ex] & \xlongequal{\text{Step 2}} \frac{ \color{purple}{ 4a^2-36 } }{ 24 } - \frac{ \color{purple}{ 192a } }{ 24 }=\frac{ \color{purple}{ 4a^2-192a-36 } }{ 24 } \end{aligned} $$