Subtract $ \dfrac{1}{9x} $ from $ \dfrac{5}{2x} $ to get $ \dfrac{ \color{purple}{ 43x } }{ 18x^2 }$.

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}{ 9x }$ and the second by $\color{blue}{ 2x }$.

$$ \begin{aligned} \frac{5}{2x} - \frac{1}{9x} & = \frac{ 5 \cdot \color{blue}{ 9x }}{ 2x \cdot \color{blue}{ 9x }} - \frac{ 1 \cdot \color{blue}{ 2x }}{ 9x \cdot \color{blue}{ 2x }} = \\[1ex] &=\frac{ \color{purple}{ 45x } }{ 18x^2 } - \frac{ \color{purple}{ 2x } }{ 18x^2 }=\frac{ \color{purple}{ 43x } }{ 18x^2 } \end{aligned} $$