Answer:
(x) = 
Step-by-step explanation:
let y = f(x) and rearrange making x the subject, that is
y = 2x + 1 ( subtract 1 from both sides )
y - 1 = 2 ( divide both sides by 2 )
= x
Change y back into terms of x and let x =
(x), thus
(x ) = 
Corresponding sides have the same ratio, so
x/15 = 12/(12+8) = 3/5
x = 15*3/5
x = 9