Work is NOT done by a person when she? pulls a sled across the ice covered backyard
lifts a book up off a desk
pushes on a desk to move it 3 m forward
pushes on a building

I think it is pulling the sled off the ice covered back yard.

firearm identification is the process of analyzing the bullets and cartridge cases left at a crime scene to determine if they came from a particular firearm. marks on the bullets and cartridges may be common to every firearm of that type. these are called class characteristics.

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.