A firm's total cost (TC) function follows a cubic function: TC(x) = 4 + 36x - 6x2 + x3 where x > 0 is the number of units the firm produces. The firm can sell each unit that it produces at a price of 20,613. How many units should the firm produce if it wants to maximize its profit where profit is total revenue minus total cost? Your answer can contain decimals.