Domain is the x values.
x = {0, 2, 4, 6, 8}
Range is the y values.
y = {8, 6, 4, 2, 0}
You can tell if it is a function if every input value (x) has only one output value (y). For example, you cannot have a function if two points with the x value of 0 had y values of 2.
In this set of points, every x value has one unique y value, so it is a function.
