I will upvote immediately. I have figured out A and c but I need help of b and d

Say we have the message 3 9 2 7 a) Give the checksum we would get using the last digit of sum checksum (show work) b) Give a sequence with two errors that produces the same checksum (show work) c) Give the staircase checksum for the original message (show work) d) Give the staircase checksum for the erroneous sequence from part b (show work)