Momentum of first box \(=\) Momentum of second box
\(m _1 v _1= m _2 v _2\)
\(5 \times 7=2 \times v _2\)
\(v _2=15.5\,m / s\)
\(\text { Velocity of center of mass }\)
\(v _{ cm }=\frac{\left( m _1 v _1\right)+\left( m _2 v _2\right)}{ m _1+ m _2}\)
\(v _{ cm }=\frac{(5 \times 7)+(2 \times 15.5)}{5+2}\)
\(v _{ cm }=10\,m / s\)