Answer: -2.9 m/s
Explanation:
If we assume that no external forces act during the collision, total momentum must be conserved.
If the collision is completely inelastic, so both masses continue stuck together after it, we can consider that the system behaves like it were a single mass, with a single speed.
So, we can write the following:
p₁ = p₂ ⇒ m₁.v₁ + m₂.v₂ = (m₁ + m₂) vf
Replacing by the values, and solving for vf, we get:
vf = (19 kg. 1.5 m/s + (8 Kg. (-13.5 m/s)) / 19 kg + 8 Kg = -2.9 m/s.