Use conservation of momentum. (or Newtons first law)
and also use conservation of mass
Assume elastic collision
m1v1 + m2v2 = m1V1 + m2V2
if we move things over and group like terms
m1(v1 - V1) = m2(V2 - v2)
m1 = 4kg
m2 = 1kg
v1 = 6.5m/s
v2 = 0
V1 = 4.8
V2 = ?
m1(v1 - V1) = m2(V2 - v2)
4(6.5-4.8) = 1(x-0)
(expand and group)
x = 6.8m/s
(Other ways to do this!)
