Show that $2^{2046} ≡ 1 \pmod {2047}$. Check whether 2047 is prime.
We start with $2^{11} = 2048$, and so
$ 2^{11} \equiv 1 \pmod {2047} $
$ 2^{2046} \equiv (2^{11})^{186} \equiv 1 \pmod {2047}$.
2047 is not prime since $2047 = 23 \times 89$.