Question

Let angle 1, angle 2, and angle 3 have the following relationships.

Answer 1

180 degrees

Vertical angles are opposite angles that share only a vertex. Since ∠3 is adjacent to both ∠1 and ∠2, this means that ∠3 shares a side and vertex with both of these angles.
This means that ∠3 and ∠1 form a straight line; this makes them a linear pair, which makes their sum 180°.