Answer:
(after two seconds)
(with no acceleration)
Explanation:
Givens




(constant)
(friction)

First, we have to find the speed after 2 seconds, when its initial speed is
. The acceleration can be found by using Newton's Second Law
Vertical forces:

Where
, which is deducted from the right angle formed (refer to the image attached).
Then,

Now, we solve for 

Then, we use the following formula to find the speed after 2 seconds

Now, if the friction force is
and the box falls with constant speed, what would be the magnitude of
to happen.
In this case, the sum of vertical forces would be

Where 
Notice that the net vertical force is zero, because there's no acceleratio, the box is moving at a constant speed.

Then, we solve for 

Therefore, the force needed to have no acceleration is 24.5 N.