Need help with permutations/combinations

Determine the size of each collection. You may use binomial coeicients, multiset coef- cients, Stirling numbers, and permutation numbers in your answer. Give a brief explanation, but you do not need to give a detailed proof. (a) The number of sequences of length 2021 using the 26 letters of the alphabet so that each letter appears at least once. (b) The number of ways of writing the number 2021 as a sum of 5 positive integers, each of which is least 100