Question

What is the counter example for n-0=0-n

Answer 1

uh, well, n could be anything, really... let's pick n = 1

1-0 does not equal 0-1

Answer 2

The equation you have, n - 0 = 0 - n only works for n = 0.
We know that by solving it.
n - 0 = 0 - n
n = -n
2n = 0
n = 0
That means any value other than n = 0 is a counterexample.

Answer 3

^ right, I didn't consider n = 0