Use the distributive property.

Subtract $ \dfrac{i}{4} $ from $ \dfrac{-i}{2} $ to get $ \dfrac{ \color{purple}{ -3i } }{ 4 }$.

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

$$ \begin{aligned} \frac{-i}{2} - \frac{i}{4} & = \frac{ \left( -i \right) \cdot \color{blue}{ 2 }}{ 2 \cdot \color{blue}{ 2 }} - \frac{ i }{ 4 } = \\[1ex] &=\frac{ \color{purple}{ -2i } }{ 4 } - \frac{ \color{purple}{ i } }{ 4 }=\frac{ \color{purple}{ -3i } }{ 4 } \end{aligned} $$

Multiply $ \dfrac{-3i}{4} $ by $ 5 $ to get $ \dfrac{ -15i }{ 4 } $.

Step 1: Write $ 5 $ as a fraction by putting $ \color{red}{ 1 } $ in the denominator.

Step 2: Multiply numerators and denominators.

Step 3: Simplify numerator and denominator.

$$ \begin{aligned} \frac{-3i}{4} \cdot 5 & \xlongequal{\text{Step 1}} \frac{-3i}{4} \cdot \frac{5}{\color{red}{1}} \xlongequal{\text{Step 2}} \frac{ \left( -3i \right) \cdot 5 }{ 4 \cdot 1 } = \\[1ex] & \xlongequal{\text{Step 3}} \frac{ -15i }{ 4 } \end{aligned} $$