How to decrypt block cipher

I have come across a interesting question, which is to give a decryption formula in the condition where entity only know the key and ciphertext to the following encryption function, C=E(Key,Nonce||Plaintext). Where the nonce is random and concatenate to the plaintext so that the nonce (half of the block) and plaintext (half of the block)is exactly one block long. What theories or concept is it applying?