@vinsan,
Here is a variant of the argument given by ''engineer'' ;
Any two consecutive integer multiples of 3 differ by exactly 3 , example:-
.... -9 , -6, -3 , 0 , 3 , 6 ,9 .... etc. Now if x is an integer then x-1 , x , x+1 are three consecutive integers and so exactly one of them is divisible by 3 . Now as given x is not divisible by 3 , so exactly one of x-1 , x+1 is divisible
by 3 i.e. (x - 1) (x+1) is divisible by 3 . Now we write x^2 + 8 simply as
x^2 + 8 = x^2 - 1 +9 = (x - 1) (x+1) + 9 , and since 9 is divisible by 3 we get
x^2 + 8 is divisible by 3 for any integer x which is not divisible by 3 .