Explanation

To get a feel for this information, let's first pick a random value for x.
Let's say x = 2
The given info tells us that f(x) = f(x + 1) for all values of x
So, f(2) = f(2 + 1)
In other words, f(2) = f(3)

Let's try a different value of x.
How about x = 3
The given info tells us that f(x) = f(x + 1) for all values of x
So, f(3) = f(3 + 1)
In other words, f(3) = f(4)

And so on.

We can see that f(2) = f(3) = f(4) = f(5) and so on
So, if f(4) = 17, we can be certain that f(8) = 17 as well.

Answer:

Cheers,
Brent