You can put this solution on YOUR website! you will have to define opposite. do you mean opposite signs? So the opposite of 2 is -2?
Assuming this is what opposite means, there will be no counterexample. Let p be some whole number, then the opposite of p is -p = -1*p. Since both -1 and p are whole numbers (we know p is whole because that's what we assumed in the first place when picking p), -p will be whole as well