Use the distributive property.

Combine like terms

Multiply $ \dfrac{15}{4} $ by $ x $ to get $ \dfrac{ 15x }{ 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.

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

Subtract $3$ from $ \dfrac{15x}{4} $ to get $ \dfrac{ \color{purple}{ 15x-12 } }{ 4 }$.

Step 1: Write $ 3 $ as a fraction by putting $ \color{red}{ 1 } $ in the denominator.

Step 2: To subtract raitonal expressions, both fractions must have the same denominator.
We can create a common denominator by multiplying the second fraction by $\color{blue}{4}$.

$$ \begin{aligned} \frac{15x}{4} -3 & \xlongequal{\text{Step 1}} \frac{15x}{4} - \frac{3}{\color{red}{1}} = \frac{ 15x }{ 4 } - \frac{ 3 \cdot \color{blue}{ 4 }}{ 1 \cdot \color{blue}{ 4 }} = \\[1ex] & \xlongequal{\text{Step 2}} \frac{ \color{purple}{ 15x } }{ 4 } - \frac{ \color{purple}{ 12 } }{ 4 }=\frac{ \color{purple}{ 15x-12 } }{ 4 } \end{aligned} $$