input: Write a function to extract all the words starting with a specific letter from a given string. If the given string is empty then return an empty list.
  assert extract_words('This is a sample sentence to test the program', 's')==['sample', 'sentence']
  assert extract_words('Hello world', 'a')==[]
  assert extract_words('', 's')==[]
  assert extract_words('Sphinx of black quartz, judge my vow', 'S')==['Sphinx']
  assert extract_words('The quick brown fox jumps over the lazy dog', 'x')==['x']
output: def extract_words(s, letter):
      return [word for word in s.split() if word.lower().startswith(letter.lower())]