a model for the us average price of a pound of white sugar from 1993 to 2003 is given by the function S(t)= -0.00003237t^5 + 0.0009037t^4 -0.008956t^3 + 0.03629t^2 + 0.4074 where t is measured in years since August of 1993. estimate the times which sugar was cheapest and most expensive during the decade from 1993 to 2003,

I know i need to take the derivative and find the critical points but i am having a hard time setting the derivative to zero