Mersenne Prime Proof

well i was trying to proof that 2^p -1 can only be prime if p is also a prime.
so p=r*s

2^(r*s) - 1 = (2^r)^s-1=(2^r-1)((2^r)^(s-1)+(2^r)^(s-2)+...+2r+1)
how was this step done?
 

Aren't there a few exceptions to this?