How do you do this? can you tell me how to set up the equation.

First you divide 9 by 2 than you'll get 4.5 that's your ratios after that do                 4.5 x 4.5 x 3.14 = 63.585 than round it 63.59 that's your answer

Do pi times the radius squared...pi is 3.14, radius is half of diameter, and square is multiply it by itself