When you look at this, you see that the right side is bigger than the left, so left subtract the left from the right:
9-5=4
12-8=4
13-9=4
n-4=4
You see that? When you put in the input, the output is always greater by 4. Since you see this, you simply add 4 to the n, so the output would be;
n n+4
