domain is the sex of x values that are feasible. this is all of the positive integer values + 0, in case you only consider that can produce whole units.
range is the set of possible results for c(x), possible costs.
you can derive this from the fact that c(x) is a parabole and you can draw it, for which you can find the vertex of the parabola, the roots, the y intercept, the shape. also limit the costs to be positive.
you can substitute some values for x to help you, for example.
x y
0 200
1 200 - 7 + 0.345 = 193.345
2 200 - 14 + .345(4) = 187.38
3 200 - 21 + .345(9) = 182.105
the function does not have real roots, then the costs never decrease to 0.
the function starts at c(x) = 200, decreases until the vertex, (x = 10, c = 164.5) and starts to increase
then the range goes to 164.5 to infinity, limited to the solution for x = positive integers.
