It is a quadratic function. If you were to make a table of the x and y values of the function it would look like this:
x y
0 -4
1 -1
2 8
Just by looking at these three values, we can see that they do not increase at a consistent rate. It increases by 3 from 0 to 1, but increase by 9 from 1 to 2. You can tell if a function is linear by seeing if it increases at a constant rate. (In this case it does not)
