To rewrite the expression $-\sqrt{-25}$ as a complex number, we can use the imaginary unit $i$, where $i = \sqrt{-1}$.

1. Start with the expression: $-\sqrt{-25}$

2. Rewrite $\sqrt{-25}$ using the imaginary unit $i$: $\sqrt{-25} = \sqrt{25 \cdot (-1)} = \sqrt{25} \cdot \sqrt{-1} = 5i$

3. Now, include the negative sign from the original expression: $-\sqrt{-25} = -5i$

Thus, the expression $-\sqrt{-25}$ rewritten as a complex number is: $-5i$

Would you like more details or have any questions about this? Here are some related questions to explore further:

1. What is the value of $\sqrt{-9}$ in terms of $i$?
2. How would you express $\sqrt{-36}$ as a complex number?
3. If $z = 4 - 3i$, what is the magnitude of $z$?
4. How do you multiply two complex numbers, say $3 + 4i$ and $2 - 5i$?
5. What is the complex conjugate of $7 + 2i$?

Tip: Remember that $i^2 = -1$. This is fundamental when simplifying expressions involving imaginary numbers.