input: Write a python function to search a given word in a sentence and return the index.
  assert find_word("Python is a powerful programming language.", "powerful") == 14
  assert find_word("The quick brown fox jumps over the lazy dog", "fox") == 16
  assert find_word("Hello world", "world") == 6
  assert find_word("This is a test sentence", "test") == 10
output: def find_word(sentence, word):
      return sentence.find(word)