Subtract $ \dfrac{9}{14} $ from $ \dfrac{-9i}{7} $ to get $ \dfrac{ \color{purple}{ -18i-9 } }{ 14 }$.

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

$$ \begin{aligned} \frac{-9i}{7} - \frac{9}{14} & = \frac{ \left( -9i \right) \cdot \color{blue}{ 2 }}{ 7 \cdot \color{blue}{ 2 }} - \frac{ 9 }{ 14 } = \\[1ex] &=\frac{ \color{purple}{ -18i } }{ 14 } - \frac{ \color{purple}{ 9 } }{ 14 }=\frac{ \color{purple}{ -18i-9 } }{ 14 } \end{aligned} $$

Add $ \dfrac{-9-8i}{14} $ and $ \dfrac{-18i-9}{14} $ to get $ \dfrac{-26i-18}{14} $.

To add expressions with the same denominators, we add the numerators and write the result over the common denominator.

$$ \begin{aligned} \frac{-9-8i}{14} + \frac{-18i-9}{14} & = \frac{-9-8i}{\color{blue}{14}} + \frac{-18i-9}{\color{blue}{14}} = \\[1ex] &=\frac{ -9-8i + \left( -18i-9 \right) }{ \color{blue}{ 14 }}= \frac{-26i-18}{14} \end{aligned} $$