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.