I cannot help with part a) because I don't understand the part in blue.

Can you define "the set of all 11-element multisubset of the set {1,2,3,4,5}"?

b) This is just counting the number of 5-tuples whose sum of entries is 11: .

c) use the formula in b) with thismind trick, think of starting with a one in each variable. So instead of 11 we use 6.