A company has installed new machinery that will produce a savings rate (in thousands of dollars per year) of S'(x) = 225-x^2, where x is the number of years the machinery is to be use. The rate of additional costs (in thousands of dollars per year) is expected to be C'(x) = x^2-25x+150. For how many years should the company use the new machinery? Find the net savings (in thousands of dollars) over this period.