The TL;DR version of this is that given some $n \in \mathbb{N}, a,b \in \mathbb{C}$ we have
$$
(a+b)^n = \sum_{i=0}^n \binom{n}{i}a^ib^{n-i}
$$
where
$$
\binom{n}{i} = \frac{n!}{(n-i)!i!}
$$
and $k!$ is defined recursively as:
$$
0! = 1, (k+1)! = (k+1)k!
$$
Unsure why @Pedro didn't just post an answer, I guess I will (figured it'd be best to close this question).
Check out the Binomial Theorem.
The TL;DR version of this is that given some $n \in \mathbb{N}, a,b \in \mathbb{C}$ we have $$ (a+b)^n = \sum_{i=0}^n \binom{n}{i}a^ib^{n-i} $$ where $$ \binom{n}{i} = \frac{n!}{(n-i)!i!} $$ and $k!$ is defined recursively as: $$ 0! = 1, (k+1)! = (k+1)k! $$