If we have q= Σ exp(2*pi*j*k*n/N) , for n=0 to M -1

I want to prove a condition for M such as q=0 . .

I'm a bit confused. I start to solve it by knowing that

Σ exp(2*pi*j*k*n/N) , for n=0 to N-1 , = 1- exp(2pi*j*k)/ 1-exp(2pi*j*k/N)

...

thanks in advance

Nikolas