Given

We can sum f(x) on both sides, and we will get

But -f(x) + f(x) = 0, then

Now we can combine the terms and we get

Divide both sides by 2

And here we have solved our problem. That's the only function that is odd and even at the same time, the graph of this function is basically the x-axis because it's a constant in y = 0, we can see even and odd because

Therefore

Odd and even at the same time.