$81$ isn't a prime so I can't use Fermat's little theorem.
$81=3^{4} \Rightarrow \varphi(81)=54$ but about the $gcd(x,81)$ , I don't know if it's equals to $1$ or not
and euler theorem won't help because $x^{54}\equiv 1 \pmod{81}$ and I need to find $x^{31}$
How do I approach this problem?