Which is the inverse of the function f(x)=1/3 x+5?

Replace the f(x) with y and proceed to make x the subject:
y=1/3x+5
y-5=1/3x
We need to ÷1/3 on both sides, so ×3 on both sides
3(y-5)=x
3y-15=x
Now swap y with x and replace the isolated x with f(x) so the inverse function is:

Then just do it (lol)