How is this mathemathical "shortcut" made (dumb question)

Question

I have this expression that is being simplified but I don't understand this part:

$2^{k+1} - \sum^k_{i=0}2^i = 2^{k+1} - \sum^{k-1}_{i=0}2^i-2^k = 2^{k} - \sum^{k-1}_{i=0}2^i$

how is $2^{k+1}$ turned into $2^{k}$ when $-2^k$ is removed?

Answer 1

$2^{k+1}-2^k = 2^k (2-1)=2^k$.