Find a formula f-1(x). Give the domain of f-1, including any restrictions "inherited" from f

So one of my problem is f(x)=3x-6

But I'm kind of confused on how to start off

At first I thought it was like having the equation all go to the power of -1

So I got y=(1/3)x-(1/6)

But in the answer book it says: y=(1/3)x + 2