input: Write a function to extract all the words starting with a specific letter from a given string. Return a list of words that start with the specified letter. 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', 'start', 'specify', 'steps']
  assert extract_words("Quick brown fox jumps over the lazy dog", 'q')==['Quick']
  assert extract_words("Hello, how are you?", 'z')==[]
  assert extract_words("", 'a')==[]
  assert extract_words("All the words start with different letters", 't')==['the', 'start', 'with', 'the']
output: def extract_words(s, letter):
      return [word for word in s.split() if word.startswith(letter)]