Return to the start and average speed

A particle starts from rest at a fixed point \(O\) and moves in a straight line so that its velocity is \(v=12t-3t^2\) m/s, where \(t\) seconds is the time after leaving \(O\). Find the time at which the particle returns to \(O\), the total distance travelled in the first \(6\) seconds, and its average speed over the first \(6\) seconds.