6.6:19
6.7:36

and the following:

Question: What is the coefficient of x^2y^3z^3 in (x+y+z)^8?

(x+y+z)^8=(x+y+z)(x+y+z)...(x+y+z)   (8 times).
The coefficient of x^2y^3z^3 is the number of ways we can
select x from 2 of the terms, y from 3 of them, and z from the
remaining three; for instance xyyzxyzz corresponds to picking
x from the first, y from the second and third, and so on.
So the coefficient of x^2y^3z^3 is the number permutations
of xxyyyzzz which is 8!/2!3!3!.

In general, what is the coefficient of x^i y^j z^k (with
i+j+k=n) in (x+y+z)^n? Answer n!/i!j!k!