The only way this can happen is if x-y and x+y are choosen.Each of the choixes have the probbaility of 1/4


P(choosing x+y in the first pick) = 1/4
p(choosing x-y in the second pick ) = 1/3 (only 3 left since x+y has been picked already)

(OR)

P(choosing x-y in the first pick) ) = 1/4
P(choosing x+y in the second pick) = 1/3

So the total probabiltiy is:

1/4*1/3 + 1/4*1/3
= 1/12 + 1/12
= 2/12
=1/6