To solve this type of problem we must make a sum of the forces acting on the cart, since the forces are vectors, we know that vectors can be added, In this way forces that go in the same direction are added and forces in opposite directions are subtracted.
FL = force to the left = - 15 [N]
FR = force to the right = 5 [N]
FT = Resultant force [N]
- 15 + 5 = FT
FT = - 10 [N]
The negative sign tells us that the force acts to the left.
