1. **State the problem:** Rewrite the expression $-\sqrt{-48}$ as a complex number and simplify all radicals. 2. **Recall the imaginary unit:** The imaginary unit $i$ is defined as $i=\sqrt{-1}$. 3. **Rewrite the expression using $i$:** Since $-48$ is negative, we can write $$-\sqrt{-48} = -\sqrt{48 \times (-1)} = -\sqrt{48} \times \sqrt{-1} = -\sqrt{48} \times i.$$ 4. **Simplify $\sqrt{48}$:** Factor 48 into perfect squares: $$48 = 16 \times 3,$$ so $$\sqrt{48} = \sqrt{16 \times 3} = \sqrt{16} \times \sqrt{3} = 4\sqrt{3}.$$ 5. **Substitute back:** $$-\sqrt{-48} = -4\sqrt{3} \times i = -4i\sqrt{3}.$$ 6. **Final answer:** The expression rewritten as a complex number is $$\boxed{-4i\sqrt{3}}.$$