Solving this problem is just a matter of plugging in the value 170 for x and evaluating f(x). To do that, however, you need a calculator. So f(170) = 700 * (0.5)^(170/90) ~ 700 * (0.5)^1.8889 ~ 700 * 0.27 ~ 189. So, after 170 years, the amount of radioactive material remaining is about 189 pounds.