1. **State the problem:** We have a polygon with side lengths 8 feet, 6 feet, 7 feet, and an unknown side length $x$. The perimeter is 29 feet. 2. **Formula:** The perimeter $P$ of a polygon is the sum of all its side lengths. $$P = ext{sum of all sides}$$ 3. **Set up the equation:** $$8 + 6 + 7 + x = 29$$ 4. **Simplify the known side lengths:** $$21 + x = 29$$ 5. **Solve for $x$:** $$x = 29 - 21$$ $$x = 8$$ 6. **Answer:** The unknown side length is **8 feet**. This matches the option 8 ft.