input: Write a function to extract all the words from a given string that start with a specified letter. If no words start with the specified letter, return an empty list.
  assert extract_words('This is a sample sentence to test the program', 's')==['sample', 'sentence', 'to', 'test', 'the', 'program']
  assert extract_words('Hello World', 'H')==['Hello']
  assert extract_words('This is an example', 'z')==[]
  assert extract_words('', 'a')==[]
  assert extract_words('Testing this function', 'T')==['Testing', 'this']
output: def extract_words(s, letter):
      return [word for word in s.split() if word.startswith(letter)]