A model for the US average price of a pound of white sugar from 1993 to 2003

Question:

A model for the US average price of a pound of white sugar from 1993 to 2003 is given by the function
S(t) = - 0.00003237t5 + 0.0009037t4 - 0.008956t3
+ 0.03629t2 - 0.04458t + 0.4074
where is measured in years since August of 1993. Estimate the times when sugar was cheapest and most expensive during the period 1993-2003.