How does thermal energy play a role in particle motion and changes of state?

Increasing an object's thermal energy increases particle motion and can cause a change of state. Decreasing an object's thermal energy increases particle motion and can cause a change of state.

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.

+15-35=-20

going in opposite directions