Chapter 3 - Parallel and Perpendicular Lines - 3-3 Proving Lines Parallel - Practice and Problem-Solving Exercises - Page 163: 50

I) 19

The two labeled angles form a linear pair, because their non-common rays form a line. \begin{array} {ll} \\2x+(8x-10)=180 & \text{Definition of Linear Pair} \\10x-10=180 & \text{Combine like terms} \\10x=190 & \text{Addition Property of Equality} \\x=19 & \text{Division Property of Equality} \end{array}