a horizontal force of 290N is applied to a stationary wooden box in one direction, and a 800N horizontal force is applied in the opposite direction. What additional force is needed for the box to remain stationary?

Answer 1

F + (800-290) = 0,
F + 510 = 0,
F = - 510N.

F should be equal and opposite of the
resultant of the other 2 forces.