Chapter 1 - Solving Linear Equations - 1.2 - Solving Multi-Step Equations - Communicate Your Answer - Page 11: 5

8 sides.

For a polygon with $n$ sides, the sum of the angle measures is $S=(n-2)180^{\circ}$. Given: $S=1080^{\circ}$ $\implies 1080=(n-2)180$ Dividing the above equation by $180$ and adding $2$ on both sides, we get $\frac{1080}{180}+2=n$ or $n=8$.