Using Python

I know how to do part a but im having troubled doing part b

13a-Write a function named CountVowels that takes one parameter s(a string that may contain upper or lower cases characters). The function countVowels should return the number of vowels in s , the vowels are aeiouAEIOU.

13b-write a function named vowelFrequency that takes one parameter t, a string. The function vowelFrequency should return a dictionary in which each key is the vowel count for some word in t. The value corresponding to each key is a list of all words having that numbers of vowels. A word should appear at most one in a given list. Upper and lower cases letter are considered the same letter( YES is the same as yes).The function vowelFrequency should call the function countVowels for each word in t to find how many vowels it has.