The function is f(x) = 2x + 1. When you put in a number for x, like f(4), you're essentially saying, "I want the value of f(x) when x = 4." So to find this value you plug in 4 for x:

f(4) = 2(4) + 1 = 9

But you don't have to just put in numbers. You could put in variables and expressions too. For example, I want to know what f(y) is. Who cares whether I know what y is equal to or not? I plug in y for x:

f(y) = 2y + 1.

In your case, you want to know the value of the function when x = x + h. Plug in x + h every time you see an x:

f(x + h) = 2(x + h) + 1.

I'll give you one more example. What if I want to know the value of the function when x = ab + c?

f(ab + c) = 2(ab + c) + 1.

01