A toboggan carrying two children $($total mass $=85$kg$)$ reaches its maximum speed at the bottom of a

hill, and then glides to a stop in $21$ m along a horizontal surface. The force of friction between the toboggan and the snowy surface is $91\mathrm{.}6$N$.$Calculate the acceleration of the toboggan. HINT: Find the velocity at the end of the hill first.