1. **State the problem:** Rewrite the expression $\sqrt{-98}$ as a complex number and simplify all radicals. 2. **Recall the formula and rules:** For any negative number under a square root, use the imaginary unit $i$ where $i=\sqrt{-1}$. Thus, $\sqrt{-a} = \sqrt{a} \cdot i$ for $a > 0$. 3. **Apply the rule:** $$\sqrt{-98} = \sqrt{98} \cdot i$$ 4. **Simplify the radical $\sqrt{98}$:** Factor 98 into $49 \times 2$: $$\sqrt{98} = \sqrt{49 \times 2} = \sqrt{49} \times \sqrt{2} = 7\sqrt{2}$$ 5. **Write the final simplified complex number:** $$\sqrt{-98} = 7\sqrt{2}i$$ **Answer:** $7\sqrt{2}i$