PYTHON 3 - please help and explain..

Problem 3. Write a recursive function replace (s, a, b) that takes as arguments the strings s, a, and b and replaces all the occurrances of string a with string b in string s a) For your solution, assume that string a is only one character long. b) For your solution, assume that string a can be of any size. Use s.startswith (a), which returns True if string s starts with a and False otherwise