input: Write a function to extract all the words starting with a specific letter from a given string. The function should accept a string and a letter as input and return a list of words that start with the given letter.
  
  def extract_words(word, letter):
      words_list = []
      for w in word.split():
          if w[0] == letter:
              words_list.append(w)
      return words_list
  
  assert extract_words('This is a sample sentence to test the program', 's') == ['sample', 'sentence', 'test', 'the', 'program']
  assert extract_words('Python is an interpreted, high-level and general-purpose programming language', 'i') == ['interpreted', 'high-level', 'and', 'interpreted', 'programming', 'language']
  assert extract_words('The quick brown fox jumps over the lazy dog', 'x') == []
  assert extract_words('Functions are fundamental constructs in most programming languages', 'f') == ['Functions', 'fundamental']
  assert extract_words('', 'a') == []
output: def extract_words(word, letter):
      words_list = []
      for w in word.split():
          if w[0] == letter:
              words_list.append(w)
      return words_list