input: Write a function that takes a string and a specific letter as input, and returns a list of words that start with the given 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', 'test', 'the', 'program']
  assert extract_words('Python programming is fun', 'p')==['Python', 'programming']
  assert extract_words('Hello world, how are you?', 'w')==['world,', 'you?']
  assert extract_words('No words starting with this letter', 'z')==[]
  assert extract_words('', 'a')==[]
output: def extract_words(word, letter):
      words_list = []
      for w in word.split():
          if w[0].lower() == letter.lower():
              words_list.append(w)
      return words_list