Multiply $7$ by $ \dfrac{x}{4} $ to get $ \dfrac{ 7x }{ 4 } $.

Step 1: Write $ 7 $ as a fraction by putting $ \color{red}{1} $ in the denominator.

Step 2: Multiply numerators and denominators.

Step 3: Simplify numerator and denominator.

$$ \begin{aligned} 7 \cdot \frac{x}{4} & \xlongequal{\text{Step 1}} \frac{7}{\color{red}{1}} \cdot \frac{x}{4} \xlongequal{\text{Step 2}} \frac{ 7 \cdot x }{ 1 \cdot 4 } \xlongequal{\text{Step 3}} \frac{ 7x }{ 4 } \end{aligned} $$

Multiply $3$ by $ \dfrac{x}{8} $ to get $ \dfrac{ 3x }{ 8 } $.

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

Step 2: Multiply numerators and denominators.

Step 3: Simplify numerator and denominator.

$$ \begin{aligned} 3 \cdot \frac{x}{8} & \xlongequal{\text{Step 1}} \frac{3}{\color{red}{1}} \cdot \frac{x}{8} \xlongequal{\text{Step 2}} \frac{ 3 \cdot x }{ 1 \cdot 8 } \xlongequal{\text{Step 3}} \frac{ 3x }{ 8 } \end{aligned} $$

Subtract $ \dfrac{3x}{8} $ from $ \dfrac{7x}{4} $ to get $ \dfrac{ \color{purple}{ 11x } }{ 8 }$.

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

$$ \begin{aligned} \frac{7x}{4} - \frac{3x}{8} & = \frac{ 7x \cdot \color{blue}{ 2 }}{ 4 \cdot \color{blue}{ 2 }} - \frac{ 3x }{ 8 } = \\[1ex] &=\frac{ \color{purple}{ 14x } }{ 8 } - \frac{ \color{purple}{ 3x } }{ 8 }=\frac{ \color{purple}{ 11x } }{ 8 } \end{aligned} $$