Another way to calculate that is to say that the object has accelerated from speed 6 m/s to 9 m/s so has a change of speed 9- 6= 3 m/s. It did that in 7 s so that is an acceleration of 3/7 m/s^2. Since "force= mass times acceleration" the Force is (4 kg)(3/7 m/s^2)= 12/7 Newtons. (Rounded to one decimal place that is 1.7 but 12/7 is exact.)

I am just using m(u- v)/r rather than your (mu- mv)/t.