The correct answer is a. 3x + 6
In order to find a compound function, you must use the first letter in the function (in this case f) and plug the second letter (g) in where you see x's.
f(x) = x + 2
f(g(x)) = (g(x)) + 2
f(g(x)) = (3x + 4) + 2
f(g(x)) = 3x + 6