Dynamics doubt

I did the a) part but I could not do the b) part. This relates the law of conservation of momentum. Anyone, suggestions please!

You've found the initial velocity of the recoil. It goes to rest after moving 2cm. What is the acceleration of the rifle as it loses velocity?

Hence, what is the force acting on it? (F=ma)

Check whether it is correct:
My working:

v=0 m/s
u=2.08 m/s
s=0.02 m
a=?

v²= u²+2as
0=4.3264 + 2as
2as= -4.3264
a= -108.16

F=ma
F= 6 x 108.16
F=648.96
F=649N (to 3s.f)

I checked the answer in the book. It is different. It is 0.651kN or 651N. Why didn't my answer match?

That will be down to a rounding error most likely. What was the exact value for velocity that you got from part (a) (not the rounded version of 2.08m/s)? You should leave answers to the stated accuracy, but never round off early in calculations, even if you rounded to give an answer to the first part of a question.

eg
If initial velocity was actually 2.083, but you rounded to 2.08:
$F = 6 \times \dfrac{2.083^2}{2\times0.02} = 650.833...N = 651 N$