input: Write a function to extract all the words starting with a specific letter from a given string and return them as a list. If no words start with the specified letter, 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', 'the', 'program']
  assert extract_words('Python programming is fun', 'p')==['programming']
  assert extract_words('Hello World', 'z')==[]
  assert extract_words('Yellow Yaks', 'Y')==['Yellow', 'Yaks']
  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