input: Write a function to extract all the words from a given string that start with a specific letter. The function should take two parameters: the first parameter is a string containing the words, and the second parameter is a single character that represents the starting letter. The function should return a list of words that start with the specified letter.
  assert extract_words('This is a sample sentence to test the program', 's')==['sample', 'sentence', 'specific', 'start', 'specified']
  assert extract_words('The quick brown fox jumps over the lazy dog', 't')==['The', 'the']
  assert extract_words('Python is a powerful programming language', 'p')==['Python', 'programming', 'powerful']
  assert extract_words('Hello world', 'x')==[]
  assert extract_words('', 'a')==[]
output: def extract_words(s, letter):
      return [word for word in s.split() if word.startswith(letter)]