Answer Happy

PLEASE complete step A. and step B. and explain every step. The
other answers on answers are not helping me so please don't just copy
and paste from another answer. Thank you, I will upvote.
Q2 (MAC): Suppose Alice has four blocks of plaintext, PO, P1, P2, P3. She computes a MAC using key K1, and then CBC encrypts the data using key K2 to obtain CO, CI, C2, C3. Alice sends the IV, the ciphertext, and the MAC to Bob. Trudy intercepts the message and replaces C1 with X so that Bob receives IV, CO, X, C2, C3, and the MAC. Bob attempts to verify the integrity of the data by decrypting (using key K2) and then computing a MAC (using key K1) on the putative plaintext. a. Show that Bob will detect Trudy's tampering. b. Suppose that Alice and Bob only share a single symmetric key K. They agree to let K1=K and K2 =K=Y, where Y is known to Alice, Bob, and Trudy. Assuming Alice and Bob use the same scheme as above, does this create any security problem?