## Geometry: Common Core (15th Edition)

70$^{\circ}$ Adjacent angles forming a straight angle are supplementary
Looking at the diagram, you see that $\angle$3 and the 110$^{\circ}$ angle form a straight line. These angles are supplementary angles, meaning they add up to 180 degrees. So 180$^{\circ}$-110$^{\circ}$=70$^{\circ}$