The sum part of the formula for a signals power
A more general expression states that for $ M \geq N$:

$$ \sum_{n= N}^{n = M} c =  (M-N+1) \cdot c $$

where the derivation simply relies on fact that the epxression has (M-N+1) terms :

$$ \sum_{n= N}^{n = M} c = \{ c + c + ... + c\} = (M-N+1) \cdot c $$

And when applied for your particular case (with $N = -M$) it becomes:
$$ \sum_{n= -M}^{n = M} c = (M-(-M)+1) \cdot c = (2M+1) \cdot c $$