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 match the criteria.
  
  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', 'the']
  assert extract_words('Python programming is fun and powerful', 'p')==['Python', 'programming', 'powerful']
  assert extract_words('All good things must come to an end', 'a')==['All']
  assert extract_words('No words start with this letter here', 'x')==[]
  assert extract_words('', 'a')==[]
output: def extract_words(word, letter):
      words_list = []
      for w in word.split():
          if w[0] == letter:
              words_list.append(w)
      return words_list