Subtract $1$ from $ \dfrac{x^2}{9} $ to get $ \dfrac{ \color{purple}{ x^2-9 } }{ 9 }$.

Step 1: Write $ 1 $ 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 $\color{blue}{9}$.

$$ \begin{aligned} \frac{x^2}{9} -1 & \xlongequal{\text{Step 1}} \frac{x^2}{9} - \frac{1}{\color{red}{1}} = \frac{ x^2 }{ 9 } - \frac{ 1 \cdot \color{blue}{ 9 }}{ 1 \cdot \color{blue}{ 9 }} = \\[1ex] & \xlongequal{\text{Step 2}} \frac{ \color{purple}{ x^2 } }{ 9 } - \frac{ \color{purple}{ 9 } }{ 9 }=\frac{ \color{purple}{ x^2-9 } }{ 9 } \end{aligned} $$