In 2009, the average price of a laptop was $803 and the average price of a desktop PC was $693.

In 2009, the average price of a laptop was $803 and the average price of a desktop PC was $693. The average price of a laptop has decreased by about $143 per year and the average price of a PC has decreased by about $78. Let L(t) and D(t) be the average prices (in dollars) of laptops and desktop PC's, respectively, both at t years since 2009. 

Find equations of L and D