Subtract $ \dfrac{7}{5w} $ from $ \dfrac{9}{4w} $ to get $ \dfrac{ \color{purple}{ 17w } }{ 20w^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}{ 5w }$ and the second by $\color{blue}{ 4w }$.

$$ \begin{aligned} \frac{9}{4w} - \frac{7}{5w} & = \frac{ 9 \cdot \color{blue}{ 5w }}{ 4w \cdot \color{blue}{ 5w }} - \frac{ 7 \cdot \color{blue}{ 4w }}{ 5w \cdot \color{blue}{ 4w }} = \\[1ex] &=\frac{ \color{purple}{ 45w } }{ 20w^2 } - \frac{ \color{purple}{ 28w } }{ 20w^2 }=\frac{ \color{purple}{ 17w } }{ 20w^2 } \end{aligned} $$