If a force has a negative value, such as -100 n, that means the force
a. is less than 100 n in strength
b. acts in the opposite direction from a + 100 n force
c. is a normal force

i cant see the choices man

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.