Answer: 180 degrees
Explanation:
If there are two parallel lines such as we could see in the problem, you can basically carry some angles to the other corners if these corners show the same direction.
In this case, you can carry "a" angle to the corner which is at the top of the "b" angle. Then, you can notice that a and b angles are complementary to 180 degrees for each other. So, this is the answer.
Note: Actually, you can also realize that these two corners at the intersections of the lines should have the same angle otherwise there would be no parallelism.