Vertical angles are 2 angles which are vertical and have the same degree. For example, if an angle which is vertical to another is 80°, the other angle is 80° too. The reason why you can't say it's 50 is because they want you to identify the equation of it, so you can't really like shortcut through and you need to make an answer for explanation. Hence 2x = 100 is the best answer.
