Given the first derivative of the volume of water in a pool in a period of 240 hours is

(1/3000)x^2 + (1/15)x - 2

where the volume at the beginning of the study is 125 m^3. Daud predicts, to the nearest hour, that the volume of water in the pool is the lowest after 36 hours. Prove with calculations that Daud's prediction is correct.?