Dear student, mass of bullet, m1= 200 g = 0.2 kg

mass of gun, m2= 1 kg

velocity of bullet, V1= 5 m/s

velocity of GUN, V2= ?

According to law of conservation of linear momentum,

Linear momentum=Linear momentum

before firing                      after firing

=>.      0 = m1 V1 + m2 V2

=>.       V2 = - m1 V1/m2

=>.        - 0.2×5×1

=>.        V2 = - 1 m/s

Therefore, velocity of the gun = -1 m/s.

All the best

Hope this helps