P(x) = -x3 + 12x2 - 36x + 400, x ≥ 3 is an approximation to the total profit (in thousands of dollars) from the sale of x hundred thousand tires. Find the number of hundred thousands of tires that must be sold to maximize profit.
◦ 7 hundred thousand
◦ 6 hundred thousand
◦ 2 hundred thousand
◦ 3 hundred thousand