Domain is basically your x values. In this case, you only have 0, 1 and 2, so you use {} brackets to denote that it's just those numbers and nothing in between.
The domain is the set of X values, which are the first number in each set of parenthesis. If there is more than one identical value, you don't need to list them all, just once.
{(0, –1), (1, –2), (1, 0), (1, 2), (2, 1)}
The X values are 0 , 1 , 1, 1, 2
There are three 1's, but it only needs to be listed once.
Explanation: Look at the highest degree in the polynomial. The highest degree in the given polynomial is 4. (The 5x^4 contains the highest degree which is, of course, 4.) I hope this helps.