12. Polluted water flows into a pond. The concentration of pollutant, A c, in the pond at time t minutes is modelled by the equation c(t) = 9 - 90 000 1 10 000 + 3t) , where c is measured in kilograms per cubic metre. a) When will the concentration of pollutant in the pond reach 6 kg/m?? b) What will happen to the concentration of pollutant over time