The profit of P(x) of a cosmetics company, in thousands of dollars, is given by P(x)=-5^2+400x-2550, where x is the amount spent on advertising, in...

The profit of P(x) of a cosmetics company, in thousands of dollars, is given by P(x)=-5^2+400x-2550, where x is the amount spent on advertising, in thousands of dollars.a) what amount must be spent on advertising to obtain a profit of at least $ 4000000? Answer is : $22971 But I didn't know how they got the answer?