input: Write a function to extract all the words starting with a specific letter from a given string. If no words start with the specified letter, return an empty list.
  assert extract_words("This is a sample sentence to test the program", 's')==['sample', 'sentence', 'test']
  assert extract_words("This is a sample sentence to test the program", 't')==['This', 'to', 'test', 'the', 'the']
  assert extract_words("This is a sample sentence to test the program", 'z')==[]
  assert extract_words("", 's')==[]
  assert extract_words("Python is a programming language", 'P')==['Python']
output: def extract_words(s, letter):
      return [word for word in s.split() if word.lower().startswith(letter.lower())]