We first need to find f(3). 
f(3) = 2(3) + 8
f(3) = 6 + 8
f(3) = 14
We now want g(f(3)). 
We know that f(3) is 14. 
We also know that g(x) is 5.
As you know, 5 is a constant. No matter what the value of x is for g(x), it will not change the value of g(x). 
So, g(f(3)) = g(14) = 5.
Answer: g(f(3)) = 5.