I think what you're looking for is the inverse of the function.
First, let's talk about notation:
f(x) =

x - 6 is the same as
y =

x - 6
To find the inverse of the function, you do the following steps.
1. write the function in terms of x and y:
y =

x - 6
2. Rewrite this equation switching the variables x and y
x =

y - 6
3. Solve this equation for y: y = 2(x + 6)