where did the 4 come from? I also dont understand where the -8/5 came from. Why iss 400r not squared but -8/5r^2 is?

Can i get a step by step with in depth explaination please? I have an exam tomorow and I am so scared :(

Thank you!

2. [15 points] I have $4000 with which I'd like to build a fence in the shape of a sector, with two internal partitions each running from the arc to the opposite comer, as shown below. The outside fence costs $5 per foot, and the partitions cost $3 per foot. 9 r What is the maximum possible total area inside the fence