Wonderful! We must subtract the initial momentum (\(500 \times 4\)) from the initial momentum (\(500 \times 8\)), and divide by the time \(5\).
As you can see, in this case, the force is negative as \(\left(500 \times 5 \right) - \left(500 \times 8 \right)\) \(= -1500\) is negative. Earlier, the force was positive as the final velocity was more than the initial velocity. In other words, a positive force increases the momentum, whereas a negative force decreases the momentum.