Technicians working for the Ministry of Natural Resources have found that the amount of a pollutant in a certain river can be represented by , , where is the time (in years) since a clean-up campaign started. At what time was the pollutant at its lowest level?

Therefore, the pollution in 0.000240991 year was at its lowest level of 0.963

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Textbook answer: 0.049 years

Did I take the first derivative wrong? I checked it and I don't see anything wrong with it. Please show me my mistake and fix it for me?