input: Write a function to extract all the words starting with a specific letter from a given string. The function should accept two parameters: a string containing words separated by spaces and a letter. Return a list of words that start with the specified letter. 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', 'start']
  assert extract_words('Python programming is fun', 'p') == ['Python', 'programming']
  assert extract_words('Artificial Intelligence', 'z') == []
  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