Prove that for every symmetric bargaining game s d the

Question

Let ? be a solution concept (for F) satisfying symmetry and efficiency.

Prove that for every symmetric bargaining game (S, d), the outcome ?(S, d) is the highest point on the line x₁ = _x2 that is also in S (in other words, the point in S and on the line x₁ = x₂ for which x₁ is maximal).