1. The problem asks us to rewrite the expression $20 - \sqrt{-19}$ as a complex number using the imaginary unit $i$. 2. Recall that the imaginary unit $i$ is defined as $i = \sqrt{-1}$. 3. Using this definition, we can rewrite the square root of a negative number as: $$\sqrt{-19} = \sqrt{19 \times -1} = \sqrt{19} \times \sqrt{-1} = \sqrt{19}i$$ 4. Substitute this back into the original expression: $$20 - \sqrt{-19} = 20 - \sqrt{19}i$$ 5. The expression is now in the form of a complex number $a + bi$ where $a = 20$ and $b = -\sqrt{19}$. 6. Since $\sqrt{19}$ is already simplified (19 is a prime number), this is the simplest form. **Final answer:** $$20 - \sqrt{19}i$$