input: Write a function to extract all the words starting with a specific letter from a given string. Return a list of words that meet the criteria. If no words start with the given letter, return an empty list.
  
  def extract_words(text, letter):
      words_list = []
      for word in text.split():
          if word.startswith(letter):
              words_list.append(word)
      return words_list
  
  assert extract_words('This is a sample sentence to test the program', 's')==['sample', 'sentence', 'to', 'test']
  assert extract_words('The quick brown fox jumps over the lazy dog', 't')==['The', 'the']
  assert extract_words('Python programming is fun', 'a')==[]
  assert extract_words('', 's')==[]
output: def extract_words(text, letter):
      words_list = []
      for word in text.split():
          if word.startswith(letter):
              words_list.append(word)
      return words_list