A sled on ice moves in the ways described in the questions below. Friction is so small that it can be ignored

Given a certain formula, you simply need to plug in the three variables available to find the fourth, time, that you need.

V(final) = V(initial) + a (acceleration) × t (time)

Vf = 16m/s
Vi = = 6.5m/s
a = 3.6 m/s^2

16 = 6.5 + 3.6 × t

9.5 = 3.6 × t

t (time) = 2.64 seconds