Question

If f(g(x))=x and f(x) = 3x – 4, find g(x)

Answer 1

$f(g(x))=x;\ f(x)=3x-4\\\\therefore3[g(x)]-4=x\ \ \ |add\ 4\ to\ both\ sides\\\\3[g(x)]=x+4\ \ \ \ |divide\ both\ sides\ by\ 3\\\\g(x)=\dfrac{x+4}{3}\\\\Answer:\boxed{B.\ g(x)=\dfrac{x+4}{3}}$

Answer 2

We can do reverse engineering to identify the function or expression of g(x). we can try each function as g(x).

a. f(g(x)) = 3*( (x-4)/3) – 4 = x-4-4 = x -8
b. f(g(x)) = 3*( (x+4)/3) – 4  = x+ 4 -4 = x

Hence the answer should be B