Gene: I don't think the poster has to find the equation of a line that passes through the given points. Those points are the function, and the poster is supposed to give the domain and the range of the function. (This is a fairly common exercise any more.)
Of course, the poster then agreed with you that the function is in fact a parabola, which wasn't included in the original post, so I could be mistaken.
tinyniel: What you'd posted originally was one sort of exercise, but your reply to Gene makes it seem that this is instead a very different sort of exercise. Please reply with the full text of the exercise, including a detailed description of any associated graphic (such as the parabola whose vertex you have verified to Gene).
Thank you.
Eliz.