## Geometry: Common Core (15th Edition)

The same-side interior angles postulate tells us that $\angle4$ and $\angle7$ are supplementary. $m\angle4+m\angle7=180\longrightarrow$ definition of supplementary $70+2x=180\longrightarrow$ substitution $2x=110\longrightarrow$ subtraction property of equality $x=55\longrightarrow$ division property of equality