Correct me if I’m wrong, I believe you have to add all of those numbers together. Then divide after and plug in those remaining digits
Use some bricks and paint. paint the bricks
431.2+826.4d=2910.4
431.2 is only the first day therefore it doesn't need the variable because the variable represents the number of days. 826.4 even says "each remaining day" referring to what the variable represents.
Circumference is given by

Area is given by

figure out the radius of the circle, then use it to find area