Average acceleration is the ratio of the change in velocity to the time it takes for that change to occur:
[tex]a_{\mathrm{av}}=\dfrac{\Delta v}{\Delta t}[/tex]
We want to find [tex]\Delta t[/tex]:
[tex]6.0\,\dfrac{\mathrm m}{\mathrm s^2}=\dfrac{20.9\,\frac{\mathrm m}{\mathrm s}-3.4\,\frac{\mathrm m}{\mathrm s}}{\Delta t}[/tex]
[tex]\implies\Delta t=2.9\,\mathrm s[/tex]
