Optimization Partition Problem can be solved in polynomial time if the decision partition can be solved in polynomial time

Question

the decision partition problem is Np-complete.Now I would like to proof that if a polynomial Algorithm exists for the decision Problem than another polynomial algorithm would exist too, that would give two sets A and B that fulfill the decision Problem.Clearly this problem is Np hard .I would like a hint or something to help me to approach this problem because I am lost.How can I use the decision partition problem to output the sets A and B