Here, `m_(1)= 5kg, m_(2)=50g= 50xx10^(-3)kg`
`u_(2)= 108km//h= (108xx1000)/(60xx60s)=30m//s , u_(1)=?`
From `m_(1)u_(1)+m_(2)u_(2)=0`
`u_(1)= (-m_(2)u_(2))/(m_(1))`
`u_(1)= (-50xx10^(-3)xx30)/5= -0.3m//s`
Negative sign indicates that the gun recoils (i.e., it moves in opposite direction).