f(x) = 3x-2

if f(a)=4 determine the numerical value of f-1(4)

I'm really clueless on how to do this one.

I started with:

4 = 3x-2

x = 2

and then tried substituting that into the inverse which is y = (x+2)/3

to get: 2+2/3 = 4/3 = 1.333....

could anyone tell me if I did this correctly?