Ok so it says here

A manufacturing company finds the profit for a production level of x motors per hour is

p = 400 x sqr(12x - x^2) - 100 x e (0,12]

For what range of production is the profit

Increasing

Decreasing

Now the answer is (0, 6) increasing and (6, 12) decreasing.

When I found the derivative, I got

(-200x +2400) (12x-x^2)^-1/2

Now the critical number for the first bracket is 12, which I found, makes sence because the function decreases at 12. But I cant find where the 6 comes from.

Can anyone help?