Answer:
x is any integer not a multiple of 3
Step-by-step explanation:
This is equivalent to ...
(x mod 3)² = 1
x mod 3 = ±1 . . . . . . taking the square root
Possible values of x mod 3 are 0, 1, 2 (≅-1), so the only value excluded is 0.
x mod 3 ≠ 0 . . . . . . for any integer x
