In a function, we have the x value (the sideways line) representing the input, and the y value, which is a function of the input, or the output. This means that when you put a x value in, you get a y value in return. Therefore, the y axis (the vertical one) is a function of the x axis. Although I haven't given you the answer, you should have a pretty good idea as to what it is. Good luck, and feel free to ask any questions!