A function g is the inverse of a function f if whenever y=f(x) then x=g(y). The inverse is usually shown by putting a little "-1" after the function name, like this: f-1(y)
In our case the function is f(x)=x+2.
The inverse function of f(x) is h(x)=x-2
Multiplying this function with 9 gives: h(x)=9x-18