I have a question:
So Find the largest possible area of a rectangle having its lower left corner at the origin
(x; y) = (0; 0) and its upper right corner on the parabola y = 9 - x2.
I know how to set up the problem, the thing is, I do not know how they found out of the domain was on [0, 3]. May someone help me with that? Thank you!

Question

I have a question: 
So Find the largest possible area of a rectangle having its lower left corner at the origin
(x; y) = (0; 0) and its upper right corner on the parabola y = 9 - x2.
I know how to set up the problem, the thing is, I do not know how they found out of the domain was on [0, 3]. May someone help me with that? Thank you!

anonymous · Answer

Oh darn, that's supposed to say x^2. My fault!

anonymous · Answer

Domain is on [0,3], because 3 is the x-intercept of that parabola.

anonymous · Answer

How can you figure that out? Because I tried that and when I do it, it doesn't work.

anonymous · Answer

You set that function equal to 0, correct? 
I keep getting a negative under a root sign and that is not possible.

anonymous · Answer

$0=9-x^2$
$x^2=9$
$x=\pm 3$

anonymous · Answer

But isn't it supposed to be -9 when you transfer it to the other side?

anonymous · Answer

I didn't move the 9. I moved the -x^2

anonymous · Answer

OOOH!!! Alright, I got it! Thank you so much!