We are given an expression, and are asked wether it determines y as a function of x.
We say that the statement of "being a function of" means that a variable varies according to whatever value the other one takes on. In our case, we can clearly see that y varies according to the value that x takes: If x is 8 then y will be 2, and if x is 20 then y will be 4. Therefore, we can say that the formula determines y as a function of x