1. The problem asks to write an expression in terms of $x$ for angle $ACD$. 2. From the description, angle $ACD$ is given as $\frac{180 - x}{2}$. 3. This means the angle at point $C$ between rays $CA$ and $CD$ is half of $180 - x$ degrees. 4. The formula used here is simply the division of the difference $180 - x$ by 2, which is common in angle bisector or related geometry problems. 5. Therefore, the expression for angle $ACD$ is: $$\angle ACD = \frac{180 - x}{2}$$ This is the final answer.