Because an Adjacent Angle is when they have a common side and a common vertex (corner point) and don't overlap.
Ans the vertical angle theorem is ∠ 1 and ∠ 2 form a linear pair, so by the Supplement Postulate, they are supplementary. That is, m ∠ 1 + m ∠ 2 = 180 °. ∠ 2 and ∠ 3 form a linear pair also, so. m ∠ 2 + m ∠ 3 = 180 °.
