input: Write a function to extract all the words starting with a specific letter from a given string. The function should accept two arguments: the first argument is the input string from which to extract words, and the second argument is the specific letter to match. The function should return a list of words that start with the given letter.
  
  assert extract_words("This is a sample sentence to test the program", 's')==['sample', 'sentence', 'to', 'test', 'the', 'program']
  assert extract_words("Python is a versatile and powerful language", 'v')==['versatile']
  assert extract_words("The quick brown fox jumps over the lazy dog", 'a')==[]
  assert extract_words("Hello world", 'H')==['Hello']
  assert extract_words("", 'a')==[]
output: def extract_words(input_string, specific_letter):
      return [word for word in input_string.split() if word.startswith(specific_letter)]