Mass of the gun M = 4 kg.

Mass of bullet M = 50 g = 50 × 10^{-3}kg.

Initial velocity of the bullet v = 35ms^{-1}

Let recoil velocity of the gun be vms^{-1}

Before firing the bullet both gun and bullet were in rest so total momentum of gun and bullet is zero, After firing

Momentum of bullet = mv,

Momentum of gun = Mv

Total momentum of bullet and gun after firing = mv + Mv

Since there is no external force applied on the system.

So total momentum after firing = total momentum before firing

mv + Mv = 0

Mv = -mv

fi V = \(\frac{mv}{M}= \frac{50\times 10^{-3}\times 35}{4}\) = -0.44 ms^{-1}

Here negative sign shows that there recoil velocity of the gun is in the direction opposite to the velocity of the bullet.