Hey there was wondering if anyone could give me some help with these questions or show me a link were i could get some information to help solve them.

a) How many solutions in non-negative integers are

there to the equation:

w + x + y + z = 9 ?

b) How many solutions in positive integers?

c) Use multisets to show that there are $\displaystyle \binom{n+k-1}{k}$ terms in the multinomial expansion of (x1+x1+.....+xn)^k