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
