Chapter 2 - Solving Equations - 2-5 Literal Equations and Formulas - Practice and Problem-Solving Exercises - Page 113: 42

The polygon has $9$ sides.

In order to answer the problem, we must use the formula provided and plug in the polygon's interior angle of $1260$ degrees for $s$ and derive $n$ afterward. $s=180(n−2)\\ 1260=180(n−2)\\ \frac{1260}{180}=\frac{180}{180}(n−2)\\ 7=1\cdot(n−2)\\ 7=n−2\\ 7+2=n−2+2\\ 9=n$