1. **State the problem:** Rewrite the expression $-\sqrt{-36}$ as a complex number and simplify all radicals. 2. **Recall the definition of the imaginary unit:** The imaginary unit $i$ is defined as $i = \sqrt{-1}$. 3. **Rewrite the square root of a negative number:** $$\sqrt{-36} = \sqrt{36 \times -1} = \sqrt{36} \times \sqrt{-1} = 6i$$ 4. **Apply the negative sign outside the root:** $$-\sqrt{-36} = -6i$$ 5. **Final answer:** The expression rewritten as a complex number is $-6i$. This means the original expression simplifies to a purely imaginary number with magnitude 6 and negative sign.