I need help, please and thank you. I know that you have to multiply the diameter by 2 to find the radius, but with the 9 pi, it makes me unsure if I'm doing this correctly.

r=6

Step-by-step explanation:

The area of a circle is given by πr². Note that this uses the radius, not the diameter.

Now, one quarter of this area (ABC) is given to be 9π.

So apparently:

If you solve this:

(divide both sides by π)

(multiply both sides by 4)

so

r = √36 = 6

so the radius is 6.