a) look for:

5000 = 15t^3 + 120t^2 + 450t + 3600

Solve for t. Then use this value in p'(t)

b) Find the number of fish at the current time, that is when t = 0. Let that amount by Y

Double it, and solve for Y = 15t^3 + 120t^2 + 450t + 3600

Then, use that time in p'(t).