For questions related to multinomial coefficients, a generalization of binomial coefficients.
Multinomial coefficients are a generalization of binomial coefficients, and can be used to expand a power of a sum in a manner similar to the binomial theorem.
A multinomial coefficient can be defined by
$${n \choose k_1, k_2, \dots, k_m} = \frac{n!}{k_1! k_2! \cdots k_m!}$$
The multinomial theorem states that a power of a sum can be expanded by
$$(x_1 + x_2 + \dots + x_m)^n = \sum_{k_1 + \dots + k_m = n} {n \choose k_1, \dots, k_m} \prod_{1 \le t \le m} x_t^{k_t}$$
The multinomial coefficients can be interpreted in terms of combinatorics, as well as be placed into a generalized Pascal's triangle.
Reference: Multinomial theorem.
