input: 
  def extract_words(s: str, letter: str) -> List[str]:
      """Extract all words from a given string that start with a specific letter.
      >>> extract_words('This is a sample sentence to test the program', 's')
      ['sample', 'sentence', 'test', 'statement']
      """
  
output:     words = s.split()
      result = [word for word in words if word.startswith(letter)]
      return result