Question

Sendo f(x)= 3x -1 e f (g(x)) = 6x+8, determine g(x):

Answer 1

6x+8=3.g(x)-1
6x+8+1=3g(x)
6x+9=3g(x)
g(x)=6x+9/3
g(x)=2x+3

Answer 2

f(x) = 3x - 1 
f(g(x)) = 6x + 8

Veja:
3 g(x) - 1 = 6x + 8
3 g(x) = 6x + 8 + 1
3 g(x) = 6x + 9 
g(x) = $\frac{6x + 9}{3}$