Subtract $ \dfrac{4+7j}{2+5j} $ from $ \dfrac{1}{2} $ to get $ \dfrac{ \color{purple}{ -9j-6 } }{ 10j+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}{ 5j+2 }$ and the second by $\color{blue}{ 2 }$.

$$ \begin{aligned} \frac{1}{2} - \frac{4+7j}{2+5j} & = \frac{ 1 \cdot \color{blue}{ \left( 5j+2 \right) }}{ 2 \cdot \color{blue}{ \left( 5j+2 \right) }} - \frac{ \left( 4+7j \right) \cdot \color{blue}{ 2 }}{ \left( 2+5j \right) \cdot \color{blue}{ 2 }} = \\[1ex] &=\frac{ \color{purple}{ 5j+2 } }{ 10j+4 } - \frac{ \color{purple}{ 8+14j } }{ 10j+4 }=\frac{ \color{purple}{ 5j+2 - \left( 8+14j \right) } }{ 10j+4 } = \\[1ex] &=\frac{ \color{purple}{ -9j-6 } }{ 10j+4 } \end{aligned} $$