input: Write a function to extract all the words starting with a specific letter from a given sentence. The function should return a list of words that match the criteria. If there are no words matching the criteria, the function should return an empty list.
  assert extract_words("This is a sample sentence to test the program", 's')==['sample', 'sentence', 'test']
  assert extract_words("Hello world how are you", 's')==[]
  assert extract_words("Python programming is fun and simple", 'p')==['programming']
output: def extract_words(sentence, letter):
      return [word for word in sentence.split() if word.startswith(letter)]