input: Write a function to extract all the words starting with a specific letter from a given string. The function should return a list of words that start with the specified letter.
  
  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', 'to', 'the', 'program']
  assert extract_words("Hello world", 'H') == ['Hello']
  assert extract_words("Python programming is fun", 'f') == []
  assert extract_words("", 'a') == []
  assert extract_words("Apple orange banana", 'A') == ['Apple']
output: def extract_words(word, letter):
      words_list = []
      for w in word.split():
          if w[0] == letter:
              words_list.append(w)
      return words_list