You have figured out

only is troubling you.

you have taken the correct steps

only this is that since this is retardation,To find V im using V2 = u2 + 2as (the first 2's are ment to be powers)

when s= displacement (i gaver this is recoil? so it would be 1.25cm)

u = inital velocity

v = final velocity=0

a = acceleration (A=F/M=50/0.02= 2500m/s)

t = time

giving