You define a function f(x) which gives the cost of buying x packages of cookies. You are asked for the domain of the function. That is, what values can x take on? x is the number of packages bought.
It makes no sense to buy a negative number of packages. It also makes no sense to buy 1/2 a package or 3/4 of a package as the store won’t sell you a fraction of a package. Try going to the store and buying half a package of oreo cookies. I doubt you’ll get very far :)
So it makes sense to buy 0, 1, 2, 3, 4, ... boxes of cookies. These are whole numbers. So the domain is the set of whole numbers. You could also write the domain like this {0, 1, 2, 3, ...} making sure to use the curly brackets as those denote a set.
