1. The problem asks to rewrite the expression $18 - \sqrt{-49}$ as a complex number and simplify all radicals. 2. Recall that the imaginary unit $i$ is defined as $i = \sqrt{-1}$. 3. We can rewrite $\sqrt{-49}$ as $\sqrt{49 \times -1} = \sqrt{49} \times \sqrt{-1} = 7i$. 4. Substitute back into the expression: $18 - \sqrt{-49} = 18 - 7i$. 5. The expression is now written as a complex number in the form $a + bi$, where $a = 18$ and $b = -7$. Final answer: $$18 - 7i$$