You need to remember the equation that relates the force and momentum such that:
`F = (m*v - m*u)/t`
`m` represents the mass of the object
`v` represents the final velocity of the object
`u` represents the initial velocity of the object
You need to remember that the difference mv - mv represents the change in momentum, where mv represents the final momentum and mu represents the initial momentum.
Since the problem provides the initial momentum, the force and the time, yields:
`6 = (mv - 18)/10 => 60 = mv - 18 => mv = 60 + 18 = 78Kgm/s`
Hence, evaluating the final momentum, under the given conditions, yields `mv = 78Kgm/s` .