input: 
  def linear_search(arr, x):
      """
      Perform a linear search to find the index of an element in a list.
      If the element is found, return its index; otherwise, return -1.
      >>> linear_search([10, 20, 80, 30, 60, 50, 110, 100, 130, 170], 110)
      6
      >>> linear_search([10, 20, 80, 30, 60, 50, 110, 100, 130, 170], 200)
      -1
      """
      for index, item in enumerate(arr):
          if item == x:
              return index
      return -1
  
output:     for index, item in enumerate(arr):
          if item == x:
              return index
      return -1