Question

A one-to-one function and it's inverse can be used to make information secure.
The function is used to encrypt a message, and it's inverse is used to decrypt the encrypted message. using the function f(x)=3x-1, the message "Hello" would be encrypted 23 14 35 35 44 since H corresponds to 8 and f(8)=3(8)-1=23.
I was told to make sure to give a function equation, ?
What's a function equation?