Assignment:
Between 1989 and 1997 the percentage of households with incomes of $100,000 or more (in 1997 dollars) is modeled by f(x) = .071x2 - .426x + 8.05, where x = 0 represents the year 1989. (Source; American Demographics, January 1999). Based on this model when did the percent of people in this income level reach its minimum? You do not have to graph the function.
Provide complete and step by step solution for the question and show calculations and use formulas.