10a - 3 of a =2 Please answer the best you can and take your time. So you can give me the best answer

Acircular city park has a sidewalk directly through the middle that is 111 - feet long. if each bag of fertilizer covers 50 square feet, then determine how many bags of fertilizers the parks and recreation department needs to use to cover the circular park. ignore all the sidewalks around and through the park.

What is the missing step in this proof ?

Nikolas calculated the volume of the prism. his work is shown below.