Can't find an example of this, this was in the previous final exams, and with finals only a week away, I need an explaination of a few problems from this last section of the book.

Using geometry, evaluate the double integral over D of [(32-x^2-y^2)^.5]dA over the circular disk D: x^2+y^2<=36

I know the answer is 144pi, but how do I come about this answer?

Thanks for the help, I will definitely have more questions over the next week!