Xavier Aptitude Test 2017 Paper

(c) If f(x) = ax + b
Then, f (f (x)) = a (ax + b) + b
Thus, a${}^2$x + ab +b = 9x + 8
Comparing coefficients of x and constant term, we get
a${}^2$ = 9 a = ±3 (but a, b are positive real numbers)
So, a = 3
36 + 6 = 8 ,6 = 2 → a + b = 5.