Subtract $ \dfrac{2}{x} $ from $ \dfrac{2}{x-9} $ to get $ \dfrac{ \color{purple}{ 18 } }{ x^2-9x }$.

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 $\color{blue}{ x-9 }$.

$$ \begin{aligned} \frac{2}{x-9} - \frac{2}{x} & = \frac{ 2 \cdot \color{blue}{ x }}{ \left( x-9 \right) \cdot \color{blue}{ x }} - \frac{ 2 \cdot \color{blue}{ \left( x-9 \right) }}{ x \cdot \color{blue}{ \left( x-9 \right) }} = \\[1ex] &=\frac{ \color{purple}{ 2x } }{ x^2-9x } - \frac{ \color{purple}{ 2x-18 } }{ x^2-9x }=\frac{ \color{purple}{ 2x - \left( 2x-18 \right) } }{ x^2-9x } = \\[1ex] &=\frac{ \color{purple}{ 18 } }{ x^2-9x } \end{aligned} $$