A manufacturer has been selling flashlights at $6 apiece, and at this price, consumers have been buying 3,000/month. The manufacturer wishes to increase the price & estimates that for every $1 he raises the price, 1,000 fewer flashlights will be sold each month. The manufacturer can produce the flashlights for $4 apiece. At what price should the manufacturer sell the flashlights to generate the greatest possible profit? Let x be the price increase (in dollars), find the max of the profit function.