Answer:
0.15 m/s
Step-by-step explanation;
The collision is inelastic, which means only momentum is conserved.
Now, before the collision the momentum of the system is
![p_{in\text{ital}}=(0.19\operatorname{kg})v+(0.85\operatorname{kg})\cdot0.70_{}(m/s)_{}]()
After the collision the momentum becomes
![p_{\text{fInal}}=(0.19\operatorname{kg}+0.85\operatorname{kg})\cdot4.2]()

The conservation of momentum demands that

therefore, we have
![(0.19\operatorname{kg})v+(0.85\operatorname{kg})\cdot0.70(m/s)_{}=0.728(kg\cdot m/s]()
Neglecting the units for a while gives us

subtracting 0.70 from both sides gives


finally, dividing both sides by 0.19 gives and rounding the nearest hundredth gives

Hence, the velocity of the hockey puck before the collision is 0.15 m/s.