Answer:
Speed, v = 60 m/s
Step-by-step explanation:
Given that,
Mass of the box, m = 18 kg
Time, t = 6.1 seconds
Initially the box is at rest, u = 0
Let v is the final speed of the box i.e. after 6.1 seconds. We can calculate it using first equation of motion as :

Here, a = g


v = 59.78 m/s
or
v = 60 m/s
So, after 6.1 seconds, the box is moving with a speed of 60 m/s. Hence, this is the required solution.