1. **State the problem:** We are given two intersecting lines forming angles, one angle is 80° and the adjacent angle is $x$. We need to find the value of $x$. 2. **Recall the rule:** Adjacent angles formed by two intersecting lines are supplementary, meaning their measures add up to 180°. 3. **Write the equation:** $$80^\circ + x = 180^\circ$$ 4. **Solve for $x$:** $$x = 180^\circ - 80^\circ$$ $$x = 100^\circ$$ 5. **Conclusion:** The value of the adjacent angle $x$ is 100°.