6. Problem to find the velocity - Principle of conservation of momentum

Two objects, each of mass 9 \(kg\), are moving in the same straight line but in opposite directions. The velocity of each object is 3 $m{s}^{-1}$ before the collision during which they stick together. What will be the velocity of the combined object after collision?

 

Velocity of the resulting object \(=\)  $m{s}^{-1}$.