The distance of a ship from its harbour is modelled by the function

d(t) = -3t³ + 3t²+18t where t is the time elapsed in hours since departure from the harbour.

a)Factor the time function.

b)When does the ship return to the harbour?

c)There is another zero of d(t).What is it and why is it not relevant to the problem?

d)Draw a sketch of the function where .

e)Estimate the time that the ship begins its return trip back to the harbour.