What is the definition of "gravitational potential energy"?

D. Energy stored in height differences.

APEX

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.

if the person is shown to have +90n, then ans is -10n.

if the person is shown to have -90n, then ans is +10n.