## Geometry: Common Core (15th Edition)

The angle marked $x$ and the angle marked $3y$ form a corresponding angle pair. The corresponding angle theorem tells us they are congruent. $x=3y\longrightarrow$ definition of congruent The angle marked $x$ and the angle marked $y$ form a linear pair, so they are supplementary. $x+y=180\longrightarrow$ definition of supplementary $3y+y=180\longrightarrow$ substitution $4y=180\longrightarrow$ combine like terms $y=45\longrightarrow$ division property of equality Substitute in the first equation to find the value of x. $x=3y=3(45)=135$