input: Write a function to extract all the words starting with a specific letter from a given string. The function should return a list of words that match the specified starting letter.
  def extract_words(words, letter):
      result = []
      for word in words.split():
          if word.startswith(letter):
              result.append(word)
      return result
  
  assert extract_words('This is a sample sentence to test the program', 's') == ['sample', 'sentence', 'test']
  assert extract_words('Python is a powerful language for scripting', 'p') == ['Python', 'powerful', 'programming']
  assert extract_words('The quick brown fox jumps over the lazy dog', 'z') == ['lazy']
  assert extract_words('No words starting with this letter', 'x') == []
  assert extract_words('', 'a') == []
output: def extract_words(words, letter):
      result = []
      for word in words.split():
          if word.startswith(letter):
              result.append(word)
      return result