Polynomials! SAT preparation practice help!!!

Question

The average retail price of gas in America from 1979 to 2008 can be modeled by the function

P (x) = 0.008x³ - 0.307x² + 4.38x + 25.72

Where P is the price of gas in cents per litre and x is the number of years after 1979

during which year did the price of gas reach 50¢/L

a.) 1985

b.) 1993

c.) 1988

d.) 1991

Answer 1

Subtract 1979 from each year and plug it in to see which one gives 50 cents

Answer 2

If this is really and SAT question it is a "thought" problem.

50 - 25.72 = 24.28

The quadratic term is the only term that significantly affects the price

24.28 / 4.38 = 5.543

6 years after 1979