Refer to the diagrams shown below.
Consider motion to the right as positive.
Momentum is defined as Mass x Velocity.
Therefore momentum of the system before impact is
P₁ = (1245 kg)*(29 m/s) + (2175 kg)*(-10 m/s) = 14355 (kg-m)/s
After impact, the two cars coalesce and travel together with velocity v.
The momentum after impact is
P₂ = (1245 + 2175 kg)*(v m/s) = 3420v (kg-m)/s
Because momentum is conserved, therefore
3420v = 14355
v = 4.1974 m/s
Answer: + 4.2 m/s
