Plot y = (red) and y = 18 (green) 1. Take the derivative y'(t) = -12t^2 + 20t + 8. 2. Equate it to zero: -12t^2 + 20t + 8 = 0, or, equivalently, 3t^2 - 5t - 2 = 0 3. Solve the quadratic equation = = . Take its positive root = = 2. 4. Substitute t = 2 into your polynomial P(t) to check the value of the profit P(2) = = 18. Answer. In two years.