input: Write a function to extract all the words starting with a specific letter from a given string. If the string is empty or there are no words starting with the specified letter, return an empty list.
  
  Use the function `extract_words(word, letter)` where `word` is the input string and `letter` is the specific letter to look for.
  
  assert extract_words('This is a sample sentence to test the program', 's')==['sample', 'sentence', 'test']
  assert extract_words('Python is a powerful programming language', 'p')==['Python', 'programming']
  assert extract_words('', 'a')==[]
  assert extract_words('No matching words here', 'z')==[]
  assert extract_words('SingleWord', 'S')==['SingleWord']
output: def extract_words(word, letter):
      return [w for w in word.split() if w.lower().startswith(letter.lower())]