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 match the criteria.
  
  function name: extract_words
  
  assert extract_words("This is a sample sentence to test the program", 's')==['sample', 'sentence', 'test', 'the', 'program']
  assert extract_words("She sells seashells by the seashore", 's')==['She', 'sells', 'seashells', 'seashore']
  assert extract_words("Hello World", 's')==[]
output: def extract_words(text, letter):
      return [word for word in text.split() if word.lower().startswith(letter.lower())]