input: Write a function to extract all the words starting with a specific letter from a given string. The function should return a list of words that match the criteria.
  def extract_words(text, letter):
      result = []
      for word in text.split():
          if word.startswith(letter):
              result.append(word)
      return result
  
  assert extract_words('This is a sample sentence to test the program', 's') == ['sample', 'sentence', 'test']
  assert extract_words('Python is a great programming language', 'g') == ['great', 'programming', 'language']
  assert extract_words('Hello World', 'H') == ['Hello']
  assert extract_words('No matching words here', 'z') == []
  assert extract_words('', 'a') == []
output: def extract_words(text, letter):
      result = []
      for word in text.split():
          if word.startswith(letter):
              result.append(word)
      return result