input: Write a function to extract all the words starting with a specific letter from a given string. The function should take two parameters: the string and the letter. Return a list of words that start with the specified letter.
  assert extract_words("This is a sample sentence to test the program", 's')==['sample', 'sentence', 'to', 'ssentence']
  assert extract_words("Python is a powerful programming language", 'p')==['Python', 'programming', 'panguage']
  assert extract_words("The quick brown fox jumps over the lazy dog", 't')==['The', 'the']
  assert extract_words("Hello how are you?", 'o')==[]
  assert extract_words("", 'a')==[]
output: def extract_words(s, letter):
      words = s.split()
      return [word for word in words if word.startswith(letter)]