input: Write a function to extract all the words starting with a specific letter from a given string. If there are no words starting 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']
  assert extract_words('Python is a great programming language', 'z')==[]
  assert extract_words('The quick brown fox jumps over the lazy dog', 'T')==['The']
  assert extract_words('', 'a')==[]
  assert extract_words('Hello world', 'x')==[]
output: def extract_words(s, letter):
      return [word for word in s.split() if word.startswith(letter)]