input: Write a function to extract all the words from a given string that start with a specified letter. The function should return a list of these words.
  
  def extract_words(word, letter):
      words_list = []
      for w in word.split():
          if w[0] == letter:
              words_list.append(w)
      return words_list
  
  assert extract_words("This is a sample sentence to test the program", 's')==['sample', 'sentence', 'test', 'start']
  assert extract_words("The quick brown fox jumps over the lazy dog", 'q')==['quick']
  assert extract_words("Hello world", 'a')==[]
  assert extract_words("", 'e')==[]
output: def extract_words(word, letter):
      words_list = []
      for w in word.split():
          if w.startswith(letter):
              words_list.append(w)
      return words_list