The answer is 6.
When we create an inverse equation, all of the outputs become the inputs for the new equation. Therefore any time we put in a number into f(x) for x and got out y, those would be opposite in the g(x) equation. Since when we put in 2 into f(x) we get 4 out, when we put 4 into g(x), we get 2 out. Knowing this we know the following:
f(2) = 4
g(4) = 2
So when we add those two together, we get 6.
