um of momenta = 0( Since both the cars stop)
Let m1 be the mass of the cadillac and m2 be the mass of the volkswagen.
then, v1=speed of cadillac and v2=speed of volkswagen.
Therefore, (m1)*(v1) + (m2)*(v2) = 0
Substituting,
(1000*4) + (2000*v2) = 0
4000 = - 2000 v2
Therefore, v2= - 2 m/s.