input: def extract_words(letter: str) -> str:
      """Extract all the words starting with a specific letter from a given sentence.
      >>> extract_words("s")
      ['sample', 'sentence', 'to', 'test']
      >>> extract_words("t")
      ['This', 'test']
      """
output:     sentence = "This is a sample sentence to test the function"
      words = sentence.split()
      result = [word for word in words if word.lower().startswith(letter.lower())]
      return result