input: Write a function to extract all the words starting with a specific letter from a given string. The function should take the string and the letter as input, and return a list of all words that start with the specified letter.
  assert extract_words("This is a sample sentence to test the program", 's')==['This', 'sample', 'sentence', 'the', 'the']
  assert extract_words("Hello world", 'H')==['Hello']
  assert extract_words("Python programming is fun", 'p')==['Python', 'programming', 'is']
  assert extract_words("No words starting with this letter", 'x')==[]
  assert extract_words("", 'a')==[]
output: def extract_words(s, letter):
      return [word for word in s.split() if word.lower().startswith(letter.lower())]