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.
There are 49 gallons of red paint. I figured it out by doing the following:
If you subtract the gallons of pink paint (21) from the total number of gallons (70), then you get the number of gallons of red paint (49) because 49 + 21 = 70. I hope that helps :)
