Motion In A Straight Line

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 $\alpha=1 \mathrm{~ms}^{-2}, \beta=6 \mathrm{~ms}^{-1}$ and $\gamma=5 \mathrm{~m}$. The average speed of the particle, in $\mathrm{ms}^{-1}$ from $t=0$ to $t=6 \mathrm{~s}$ is: