input: 
  from typing import List
  
  def extract_words(words: 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', 'test', 'system']
      """
      words_list = [w for w in words.split() if w.startswith(letter)]
      return words_list
  
output:     words_list = [w for w in words.split() if w.startswith(letter)]
      return words_list