0.5 s
A newton is defined as 1 kg*m/s^2 and momentum is defined as kg*m/s. We want an answer in seconds. Looking at the available units, we have 1 extra value of seconds, but it's in the denominator. Since we want it in the numerator, that means that we divide by that quantity. So:
3.0 kg*m/s / 6.0 kg*m/s^2 = 0.5 s
Therefore the force of 6.0 newtons acted upon the moving object for 0.5 seconds.
