The input x = 2 leads to more than one output. In this case, it leads to y = 20 and y = 40 simultaneously. This is the reason why we don't have a function. A function is only possible if each input x leads to exactly one y output.
Put another way: we cannot have x repeating any values if we want a function.
Side note: We could have y repeat however. So it's possible to have a function with the two points (1,5) and (3,5).
