Hello, could anyone give me some hints to do these following questions? Any help will be greatly appreciated, thanks

a) Let a and n be integers greater than 1. Prove that $\displaystyle a^n -1$ is prime only if $\displaystyle a = 2$ and n is prime.

b) Show that $\displaystyle 2^n + 1$ is prime only if n is a power of 2.