Im not sure what the answer is,, but all you need to know is that a real and good function contains like this; if you press on a 1 you get a 5, if you press on a 2 you get a 4, its just simple as long as its not like this, if it is it is not a function; if you press a 1 and get a 5 and if you press 1 again and get a 9 that not good.
