input: from typing import List
  
  def extract_words(word: str, letter: str) -> List[str]:
      """ Extract all the words starting with a specific letter
      >>> extract_words('This is a sample sentence to test the program', 's')
      ['sample', 'sentence', 'to', 'test', 'the', 'program']
      """
output:     words = word.split()
      result = [w for w in words if w.startswith(letter)]
      return result