Forced harmonic motion-Help

How do get the equation in the question?, dont know how to relate (u) into it. help

1. At time = 0, the nett force on the particle $F=0$ since it is just about to move. This is because the extension in the spring is just balancing the (limiting) frictional force due to the rough table.

2. Thereafter, the end of the spring A moves further away from the particle, and hence the extension increases so that the force due to the spring is greater than the limiting frictional force.

3. At time = $t$, the extension in the spring has changed, since end A, which is moving with speed $u$ has moved a certain distance away from the particle. What is this distance? (Hint: SUVAT)

4. However, at time = $t$, we are told that the particle has moved distance $x$ from its original position, clearly towards A.

So

a) what is the nett change in extension $e_t$ of the particle at time $t$?
b) what is the nett increase in force due to the spring, in terms of $e_t$?
c) what is the total force felt by the particle, at time = $t$?
d) what equation do you get when you apply Newton II using the info above?

for extension when tension equals friction i got µg/n^2,
That looks right.

for newtons second law, im confused how theres acceleration if theres a constant speed,

End A of the spring is being moved with constant speed - something is pulling it. However, the mass on end B is accelerating towards end A, since the spring is being stretched. It will accelerate until the extension of the spring once again is small enough to produce exactly the frictional force due to the floor.

also can you show me how to answer the questions please