12. To ensure information security, information needs to be transmitted in encrypted form, where the sender converts plaintext $\rightarrow$ ciphertext (encryption), and the receiver converts ciphertext $\rightarrow$ plaintext (decryption). It is known that the encryption rule is: plaintext $a, b, c$ corresponds to ciphertext $a+1, 2b+a, 3c-4$. For example, plaintext 1, 2, 5 corresponds to ciphertext $2, 5, 11$. If the receiver receives the ciphertext $21, 22, 23$, then the decrypted plaintext is _. $\qquad$
Reference answer: $20 ; 1 ; 9$