input: Write a function to extract all the words starting with a specific letter from a given string. If the string is empty, return an empty list.
  
  def extract_words(word, letter):
      words_list = []
      for w in word.split():
          if w.startswith(letter):
              words_list.append(w)
      return words_list
  
  assert extract_words('This is a sample sentence to test the program', 's')==['sample', 'sentence', 'strategize', 'subject']
  assert extract_words('A cat in the hat sat on the mat', 's')==['sat', 'sat']
  assert extract_words('Big Blue Bubble Incorporated', 'B')==['Big', 'Blue', 'Bubble', 'Big']
  assert extract_words('', 'a')==[]
  assert extract_words('Nothing to extract here', 'z')==[]
output: def extract_words(word, letter):
      words_list = []
      for w in word.split():
          if w.startswith(letter):
              words_list.append(w)
      return words_list