input: Write a python function to extract the author's name from a book title.
  assert extract_author("Harry Potter by J.K. Rowling") == "Rowling"
  assert extract_author("To Kill a Mockingbird by Harper Lee") == "Lee"
  assert extract_author("The Great Gatsby by F. Scott Fitzgerald") == "Fitzgerald"
output: def extract_author(title):
      parts = title.split(" by ")
      author = parts[-1]
      last_name = author.split()[-1]
      return last_name