Where's the denominator

Step 4: Multiply the denominators.
We are multiplying $(-4-5i)$ by its conjugate $(-4+5i)$. This follows the form $(a-bi)(a+bi) = a^2 + b^2$.
Here, $a=-4$ and $b=5$.

$$(-4-5i)(-4+5i) = (-4)^2 - (5i)^2$$ $$= 16 - (25i^2)$$

Substitute $i^2 = -1$:

$$= 16 - 25(-1)$$ $$= 16 + 25$$ $$= 41$$

The denominator is 41.