I agree with JJ's logic but this is a badly-posed question as it could equally well fit with a geometric sequence (or two).
eg:
- let x = 56, then the sequence is 112, 56, 28 (half each time), suggesting y = 14;
- or let x = -84, then the sequence is -28, -84, -252 (times three each time), suggesting y = -756.
Or you could go even more exotic, letting x = 42 for a sequence of 98,42,0 (decreasing by successively smaller multiples of 14), making y = -28...
I am sure that JJ's solution is the intended one, but without more information (were we told for certain that this is an arithmetic sequence?) then you could quote any value for y you liked and be able to justify it.