input: Write a function to extract all the words starting with a specific letter from a given string. Return a list containing all such words. 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', 'the']
  assert extract_words('This is a sample sentence to test the program', 't')==['This', 'to', 'test']
  assert extract_words('This is a sample sentence to test the program', 'a')==['a']
  assert extract_words('This is a sample sentence to test the program', 'z')==[]
output: def extract_words(s, letter):
      return [word for word in s.split() if word.startswith(letter)]