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', 'simple']
  assert extract_words("The quick brown fox jumps over the lazy dog", 'b')==['brown', 'box', 'bat']
  assert extract_words("The rain in Spain stays mainly in the plain", 'y')==['yay', 'yellow', 'yield']
  assert extract_words("", 'a')==[]
  assert extract_words("Hello World", 'z')==[]
output: def extract_words(s, letter):
      return [word for word in s.split() if word.startswith(letter)]