input: from typing import List
  
  def extract_words(word: str, letter: str) -> List[str]:
      """
      Extract all the words starting with a specific letter from a given string.
      
      >>> extract_words('This is a sample sentence to test the program', 's')
      ['sample', 'sentence', 'to', 'test']
      >>> extract_words('Python programming is fun and challenging', 'p')
      ['Python', 'programming']
      """
      words_list = [w for w in word.split() if w.startswith(letter)]
      return words_list
output:     words_list = [w for w in word.split() if w.lower().startswith(letter.lower())]
      return words_list