Add $ \dfrac{-3}{4} $ and $ \dfrac{2}{5} $ to get $ \dfrac{ \color{purple}{ -7 } }{ 20 }$.

To add fractions they must have the same denominator.
We can create a common denominator by multiplying the first fraction by $ \color{blue}{ 5 }$ and the second by $\color{blue}{ 4 }$.

$$ \begin{aligned} \frac{-3}{4} + \frac{2}{5} & = \frac{ \left( -3 \right) \cdot \color{blue}{ 5 }}{ 4 \cdot \color{blue}{ 5 }} + \frac{ 2 \cdot \color{blue}{ 4 }}{ 5 \cdot \color{blue}{ 4 }} = \\[1ex] &=\frac{ \color{purple}{ -15 } }{ 20 } + \frac{ \color{purple}{ 8 } }{ 20 }=\frac{ \color{purple}{ -7 } }{ 20 } \end{aligned} $$