A hammer of mass 1kg moving with a speed of 6m/s strkes a wall and come to rest in 0.1 sec.
In this question if we are asked to find the retarding force that stops the hammer then why we will divide 6 by 0.1 to get the retardation force

After the collision with the wall, the hammer moves with the same velocity but in the opposite direction.

From this point of time, the hammer came to rest in 0.1 s. The final velocity is zero and initial velocity(after the collision) is 6 m/s.
v = u + at
0 = 6 + a(0.1)
a = - 6/0.1 (retardation)

F = ma = (1 kg)(-6/0.1 m/s^2) = - 6/0.1 newtons
The " - " indicates retardation.