There are two things you can do with angles. You can 1) set them equal to each other or 2) set their sum equal to a definite angle measure (like 180 or 90).
These angles are congruent. This is because they are opposite angles, and opposite angles are congruent. So in this case, we set these equal to each other.
1) Set up: x + 104 = 3x
2) Isolate x: 2x = 104
x = 52
I got x = 52, but it is always helpful to double check work. Hope this helps!