f and g are what's called "functions of x". This means that you choose the number that takes x's place in the equation. For f(2), the number that takes x's place is 2.

If you look at g(x), it doesn't have an x at all. That means that no matter what x is, g is still 5.

When you add f to g, you are adding together what they can do to x. That means you can write the answer one of two ways: