Subtract $1$ from $ \dfrac{40}{1-j} $ to get $ \dfrac{ \color{purple}{ j+39 } }{ -j+1 }$.

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

Step 2: To subtract raitonal expressions, both fractions must have the same denominator.
We can create a common denominator by multiplying the second fraction by $\color{blue}{-j+1}$.

$$ \begin{aligned} \frac{40}{1-j} -1 & \xlongequal{\text{Step 1}} \frac{40}{1-j} - \frac{1}{\color{red}{1}} = \frac{ 40 }{ 1-j } - \frac{ 1 \cdot \color{blue}{ \left( -j+1 \right) }}{ 1 \cdot \color{blue}{ \left( -j+1 \right) }} = \\[1ex] & \xlongequal{\text{Step 2}} \frac{ \color{purple}{ 40 } }{ 1-j } - \frac{ \color{purple}{ -j+1 } }{ 1-j }=\frac{ \color{purple}{ 40 - \left( -j+1 \right) } }{ -j+1 } = \\[1ex] &=\frac{ \color{purple}{ j+39 } }{ -j+1 } \end{aligned} $$