input: 
  def extract_words(word, letter):
      """Extract all the words starting with a specific letter.
      >>> extract_words('This is a sample sentence to test the program', 's')
      ['sample', 'sentence', 'to', 'the', 'program']
      >>> extract_words('Apple Banana Cherry', 'A')
      ['Apple']
      """
      words_list = []
      for w in word.split():
          if w[0] == letter:
              words_list.append(w)
      return words_list
  
output:     words_list = []
      for w in word.split():
          if w[0] == letter:
              words_list.append(w)
      return words_list