Are the GRE geometry diagrams drawn to scale? Without doing any calculation I looked at these angles and y is clearly bigger than x, so I picked y and it's wrong.
There are very few assumptions that can be made when it comes to diagrams and the size of angles is definitely not one of them. This problem was set up to catch people who assume a bit too much. (We're not being mean, it's just a great learning opportunity!) Here's a blog that covers a bit about diagrams: