Use the distributive property.

Combine like terms

Multiply $ \dfrac{-4}{3} $ by $ x $ to get $ \dfrac{ -4x }{ 3 } $.

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.

$$ \begin{aligned} \frac{-4}{3} \cdot x & \xlongequal{\text{Step 1}} \frac{-4}{3} \cdot \frac{x}{\color{red}{1}} \xlongequal{\text{Step 2}} \frac{ \left( -4 \right) \cdot x }{ 3 \cdot 1 } = \\[1ex] & \xlongequal{\text{Step 3}} \frac{ -4x }{ 3 } \end{aligned} $$

Add $ \dfrac{-4x}{3} $ and $ 15 $ to get $ \dfrac{ \color{purple}{ -4x+45 } }{ 3 }$.

Step 1: Write $ 15 $ as a fraction by putting $ \color{red}{ 1 } $ in the denominator.

Step 2: To add raitonal expressions, both fractions must have the same denominator.
We can create a common denominator by multiplying the second fraction by $\color{blue}{3}$.

$$ \begin{aligned} \frac{-4x}{3} +15 & \xlongequal{\text{Step 1}} \frac{-4x}{3} + \frac{15}{\color{red}{1}} = \frac{ -4x }{ 3 } + \frac{ 15 \cdot \color{blue}{ 3 }}{ 1 \cdot \color{blue}{ 3 }} = \\[1ex] & \xlongequal{\text{Step 2}} \frac{ \color{purple}{ -4x } }{ 3 } + \frac{ \color{purple}{ 45 } }{ 3 }=\frac{ \color{purple}{ -4x+45 } }{ 3 } \end{aligned} $$