Diffie Hellman Exchange Confusion I am working on this question and I am wondering I have figured out the secret key, but my problem is I don't know how to use the secret key to decrypt the ciphertext.  

Thanks for the help!
Diffie Hellman(DH) is a key exchange method, it is not a encryption/decryption algorithm.
You have to use the secret key generated from DH in a symmetric cipher algorithm which is the algorithm used to create ciphertext from plaintext in the first place.

For example, lets say Alice and Bob make a DH key exchange to generate a secret key $K$, then Alice uses that key to send a message $M$ to Bob by encrypting it using an encrption algorithm such as AES. So Alice creates ciphertext $C =AES_{Enc}(M,K)$ and Alice sends $C$ to Bob. Then Bob takes ciphertext $C$ and decrypts it using the same algorithm and the same secret key that Alice used to get the message: $M =AES_{Dec}(C,K)$.