The problem gives the mass of the bullet and the gun and the velocity at which the bullet leaves the barrel of the gun. Using this it is not possible to determine the force that is exerted on the person firing the gun.
Only the momentum of the gun in the direction opposite to that in which the bullet is fired can be determined. This is done using the law of conservation of momentum; the total momentum of the gun-bullet system is 0. The momentum of the bullet when it leaves the barrel is 10*124 = 1240 g*m/s. The momentum of the gun in the reverse direction is also equal to 1240 g*m/s. As the mass of the gun is given as 600 g, the recoil velocity of the gun is equal to 1240/600 = 2.067 m/s