completed exercsie 11-1
completed exercise 11-2
completed exercise 12-1
completed exercise 12-2
completed exercise 12-3
completed exercise 12-4
completed exercise 12-5
completed exercise 12-6n
completed exercise 12-7
completed exercise 13-1
completed exercise 13-2
completed exercise 13-4
completed exercise 13-5
completed exercise 13-6
completed exercise 13-7
completed exercise 13-8
completed exercise 14-1
completed exercise 14-2
completed exercise 14-3
completed exercise 15-1