Subtract $ \dfrac{7}{4x} $ from $ \dfrac{9}{4} $ to get $ \dfrac{ \color{purple}{ 36x-28 } }{ 16x }$.

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}{ 4x }$ and the second by $\color{blue}{ 4 }$.

$$ \begin{aligned} \frac{9}{4} - \frac{7}{4x} & = \frac{ 9 \cdot \color{blue}{ 4x }}{ 4 \cdot \color{blue}{ 4x }} - \frac{ 7 \cdot \color{blue}{ 4 }}{ 4x \cdot \color{blue}{ 4 }} = \\[1ex] &=\frac{ \color{purple}{ 36x } }{ 16x } - \frac{ \color{purple}{ 28 } }{ 16x }=\frac{ \color{purple}{ 36x-28 } }{ 16x } \end{aligned} $$