Two charged objects are isolated in space. the charge on the positive object is twice the charge on the negative object. if the positive object exerts a 5 n force on the negative object, how much force does the negative object exert on the positive one?

The force exerted is proportional to the product of the charge and the electric field produced by the other particle. That is, it's the product of both charges so the force exerted on the positive charge by the negative one is also 5N. This can also be understood by Newton's 3rd Law.