Answer:
, because they are corresponding angles of parallel lines cut by a transversal.
by Vertical angles theorem.
Explanation:
From the given picture, we can see there are two parallel lines r and s intersected by a transversal line t.
Since corresponding angles of parallel lines cut by a transversal.
Therefore they must be equal.
Then,

Also, since
are vertical angles.
Vertical angles theorem says that vertically opposite angles at any cross section must be equal.
by Vertical angles theorem.