Combinatorics QUESTION 2-3

Let $A = {a_1, a_2, . . . , a_k}$ be an alphabet, and let $n_i$ denote the number of appearances of
letter $a_i$ in a word. How many words of length n in the alphabet A are there for which
$k = 3, n = 10, n_1 = n_2 + n_3$and $n_2$ is even?