During the period 1990-2002, the average cost d (in dollars) for a new domestic car and the average cost i (in dollars) for a new imported car can be modeled by d= 423.12t+14475 and i= -135.63t^2+2705.2t+12121 where t is the number of years since 1990. write an equation that gives the total average cost for domestic and imported cars as a function of the number of years since 1990.