Show that is not divisible by .
I do not know how to approach this.
But anyone reasonably familiar with congruences should be able to follow the one-liner I wrote above without any intermediate steps.
In general, in the context of an exam, you may be presented with such that gcd(a,n)=1 and be expected to recognise that Euler's theorem applies. But if is congruent to 1, 0, or -1 (mod n), then you should definitely take advantage of that to make calculations very easy. And there are other related topics you can learn about if you are interested in such things.