input: Write a function to extract all words starting with a specific letter from a given string. The function should return a list of words that match the criteria. If the input string is empty, the function should return an empty list.
  
  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']
  assert extract_words('hello world how are you', 'a') == []
  assert extract_words('', 's') == []
  assert extract_words('The quick brown fox jumps over the lazy dog', 'T') == ['The']
  assert extract_words('Are you sure about that?', 'A') == ['Are']
output: def extract_words(word, letter):
      words_list = []
      for w in word.split():
          if w[0] == letter:
              words_list.append(w)
      return words_list