Working on optimization and I don't know how to start a certain problem about worker efficiency.

An efficiency study of the first shift (from 8 A.M. to 5 P.M.) at a certain factory indicates that an average worker who arrives on the job at 8 A.M. will have assembled

Q(t)= -t^3 + 6t^2 +15t transistor radios t hours later. At what time during the shift is the worker performing most efficiently?

Thank you in advance.