Consider a particle moving along a straight line, whose position as a function of time is given by $s(t)=\alpha t^2-\beta t+\gamma$, where $\alpha=1$ ms$^{-2}$, $\beta=6$ ms$^{-1}$ and $\gamma=5$ m. The average speed of the particle, in ms$^{-1}$ from t = 0 to t = 6 s is: