Oil is being pumped continuously from a certain oil well at a rate proportional to the amount of oil left in the well; that is, dy/dt = ky, where y is the amount of oil left in the well at any time t. Initially there were 3,500,000 gallons of oil in the well, and 5 years later there were 750,000 gallons remaining. Write an equation for y, the amount of oil remaining in the well at any time t.