1. Explain how you calculate the net force in any direction on the box.
2. Suppose an upward force of 15 N is added to the box. What will be the net vertical force on the box?
3. What force could be applied to the box to make the net force in the horizontal direction zero? Explain.
4. Suppose a force of 25 N to the right is added to the box. What will be the net force to the right?

one person pulls 90n one way and the other has 100n pulling the other way.

so the net force = 90 - 100 = -10n,

or the ans can be 10n if the picture shows -90n and 100n instead.

in most common fires, oxygen is needed for an oxidation reaction to takes place. the combination of oxygen with any element as fuel can create fire and combustion will occur especially if wood, light materials and other combustible things are combine with oxygen.