Bullet of mass 0.05 kg is fired from a gun with velocity 700 m/s, find the momentum of the gun immediately after it's fired?

I used Momentum = mv = $\displaystyle -35 kg m/s$? - Here I'm taking the direction of the bullet as positive.

If the gun is brought to rest in 1s by a horizontal force that rises uniformly from 0 to Z N, then falls uniformly back to 0 again, find the value of Z.

No idea how to start this really. Was thinking to try and find the average acceleration and then use $\displaystyle F = ma$, however the acceleration isn't constant, so this wouldn't work would it?

Would love it if someone could give me a little pointer on this?

Thanks in advance