Reading check explain the relationship between pressure and volume.

Pressure is inversely proportional to the volume. Pressure multiplied by volume equals some constant . where P is the pressure of the gas, V is the volume of the gas, and k is a constant. This equation shows that, as volume increases, the pressure of the gas decreases in proportion.

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.