input: 
  def find_index(arr, x):
      """ Finds the index of the element 'x' in the list 'arr'.
      If 'x' is not found in 'arr', returns -1.
      
      >>> find_index([2, 5, 9, 15], 5)
      1
      >>> find_index([2, 5, 9, 15], 10)
      -1
      """
      for i in range(len(arr)):
          if arr[i] == x:
              return i
      return -1
  
output:     for i in range(len(arr)):
          if arr[i] == x:
              return i
      return -1