The equation for momentum is p = mv, where m is the mass of the object and v is its velocity. Through conservation of momentum, we know that the momentum of the rifle must be of the same magnitude and opposite direction to that of the bullet. Thus we can form two expressions for the respective momenta, set them equal, and solve for the recoil speed of the rifle.
718*0.0159 = vr*5.05 where vr is the rifle recoil speed.
Solving for vr = 718*0.0159/5.05 = 2.26 meters/second.
