A car dealer determines that if gasoline-electric hybrid automobiles are sold for x dollars apiece and the price of gasoline is y cents per gallon, then approximately H hybrid cars will be sold each year, where

Question

A car dealer determines that if gasoline-electric hybrid automobiles are sold for x dollars apiece and the price of gasoline is y cents per gallon, then approximately H hybrid cars will be sold each year, where
H(x,y)=6000−13x^(1/2)+2(0.1y+20)^(3/2).
She estimates that t years from now, the hybrid cars will be selling for 40000+300t dollars apiece and that gasoline will cost 300+20t^(1/2) cents per gallon. At what rate will the annual demand for hybrid cars be changing with respect to time 2 years from now?

I have no clue how to even begin this problem.