Is this prime?

Numbers in the form of $2^{n}-1$ are only prime if n is prime. Since 91 can be factorised into $13\times7$ , the number is not prime. However numbers in this form that are prime are called Mersenne Primes .

Fermat little theorem:

$a^{p-1} \equiv 1 mod p$

$2^{92-1} \equiv 1 mod 92$

$2^{91}-1 \equiv 0 mod 92$

but 92 is not a prime number.