inverse function

F(x)=x+9,where x> 0

it is easier to think if f(x)is changed to a y before we start.

y=x+9   where x>0

Now for the invers function you just swap all the x'es and y's over

x=y+9   where y>0

rearange

y=x-9    where y>0

y is greater than 0 when x>9

so 

y=x-9  where x>9

 

$$f^{-1}(x)=x-9\qquad $where $x>9$$