a) from the function p(t)=6(t), we know that p is 6t, so we plug this into it, to get 3.14 * (6t)^2, or 113.04t^2.
b) using what we got from a), we plug it in and get 113.04 * 8 * 8, or 7234.56 (use a calculator on this, or just bash it with pencil and paper).
find annual profit: $75,000/6 = $12,500
roi = annual profit/ initial investment
roi = $12,500/$15,000 or 83.3%
16 ways you can take.