input: Write a function to extract all the words starting with a specific letter from a given string. 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('This is a sample sentence to test the program', 'a') == ['a']
  assert extract_words('This is a sample sentence to test the program', 'T') == ['This']
  assert extract_words('This is a sample sentence to test the program', 'z') == []
  assert extract_words('', 's') == []
output: def extract_words(text, letter):
      return [word for word in text.split() if word.startswith(letter)]