Algebra 2 Binomial theorem

In Algebra 2>Polynomials>The binomial theorem>They are referencing combinatorics and "n choose k", when these two topics have not been discussed previous.

In addition, from my Googling and comments on Khan, it appears that combinatorics and "n choose k" are not taught until Pre-Calc.

Am I missing something here, or the module is out of place, OR there needs to be a module teaching combinatorics and "n choose k" prior to binomial theorem.

Thanks