input: Write a function to extract all the words starting with a specific letter from a given string. The function should return a list of words that meet the criteria.
  assert extract_words("This is a sample sentence to test the program", 's')==['sample', 'sentence', 'test', 'start']
  assert extract_words("Hello world", 'w')==['world']
  assert extract_words("Python is great", 'p')==['Python']
  assert extract_words("No words match", 'z')==[]
output: def extract_words(text, letter):
      return [word for word in text.split() if word.lower().startswith(letter.lower())]