Four forces are being applied to move an object to the left. force 1 is pushing to the right at 200n. force 2 is pushing to the left at 500n. force 3 is pushing to the right at 100n. force 4 is pushing to the left at 50n. what's the net force acting on this object?
a.0n
b.650n
c.850n
d.250n

The answer is D.250N 
You start at Zero since no force is being applied to the object. If you move it one way, it starts the positive force. Go another way harder, it becomes a negative force. What I did here was 0+200-500+100-50. It will come out as a negative but you could ignore that because you cannot have a completely negative force as a final result.