If you think about it this way,
Y and X could mean ANYTHING just as long as they are equal to each other.
For example: let x = 2, so 2x-2 =2
Well...set y=2, and there you have it!
Another one: let x = 3, so 2x-2 = 3
Well....set y = 3.....hopefully you get the idea.
Once again....since there are two variables are in one equation, and that equation is not in a system, ANY two values for X and Y will equal the same, but they HAVE to equal.
Hope this helps. :)