Vertical Angles are pairs of opposite angles made by intersecting lines. Graphically:
If two angles are vertical, then they are congruent, that is, they have the same measure.
In this case, we can see that angles ∠1 and ∠3 are vertical, then the measure of ∠3 is:

Also, we can see that angles ∠2 and ∠4 are vertical, then the measure of ∠4 is:
