To find the inverse, interchange the variables and solve for y.
And welcome to web2.0calc forum.
I know already that I am going to like you.
Your answer is right but you forgot the brackets. Without the brackets your answer really is not right.
x+1/8-3x = x+3x-1/8 = 4x - 0.125
this is not what you intended.
Maybe you already know this but the inverse of a function is the reflection of the graph across the line y=x
So you can always check your answer by graphing it.
Desmos is a very nice online graphing calculator.