Multiply $ \dfrac{5}{4} $ by $ x $ to get $ \dfrac{ 5x }{ 4 } $.

Step 1: Write $ x $ as a fraction by putting $ \color{red}{ 1 } $ in the denominator.

Step 2: Multiply numerators and denominators.

Step 3: Simplify numerator and denominator.

$$ \begin{aligned} \frac{5}{4} \cdot x & \xlongequal{\text{Step 1}} \frac{5}{4} \cdot \frac{x}{\color{red}{1}} \xlongequal{\text{Step 2}} \frac{ 5 \cdot x }{ 4 \cdot 1 } \xlongequal{\text{Step 3}} \frac{ 5x }{ 4 } \end{aligned} $$

Add $ \dfrac{-5}{9} $ and $ \dfrac{5x}{4} $ to get $ \dfrac{ \color{purple}{ 45x-20 } }{ 36 }$.

To add raitonal expressions, both fractions must have the same denominator.
We can create a common denominator by multiplying the first fraction by $ \color{blue}{ 4 }$ and the second by $\color{blue}{ 9 }$.

$$ \begin{aligned} \frac{-5}{9} + \frac{5x}{4} & = \frac{ \left( -5 \right) \cdot \color{blue}{ 4 }}{ 9 \cdot \color{blue}{ 4 }} + \frac{ 5x \cdot \color{blue}{ 9 }}{ 4 \cdot \color{blue}{ 9 }} = \\[1ex] &=\frac{ \color{purple}{ -20 } }{ 36 } + \frac{ \color{purple}{ 45x } }{ 36 }=\frac{ \color{purple}{ 45x-20 } }{ 36 } \end{aligned} $$